bpf: fix verification failure

On the latest 5.4, I'm seeing the following failure:

```
=== RUN   TestKillerOverride/override_helper/kprobe_(no_multi)
...
load program: invalid argument:
	; generic_tracepoint_filter(void *ctx)
	0: (bf) r6 = r1
	1: (b7) r1 = 0
	; int ret, zero = 0;
	2: (63) *(u32 *)(r10 -28) = r1
	last_idx 2 first_idx 0
	regs=2 stack=0 before 1: (b7) r1 = 0
	3: (bf) r2 = r10
	;
	4: (07) r2 += -28
	; msg = map_lookup_elem(&tp_heap, &zero);
	5: (18) r1 = 0xffff954178190400
	7: (85) call bpf_map_lookup_elem#1
	8: (bf) r7 = r0
	; if (!msg)
	9: (15) if r7 == 0x0 goto pc+1989
	 R0_w=map_value(id=0,off=0,ks=4,vs=24304,imm=0) R6_w=ctx(id=0,off=0,imm=0) R7_w=map_value(id=0,off=0,ks=4,vs=24304,imm=0) R10=fp0 fp-32=mmmm????
	; &msg->caps, &filter_map, msg->idx);
	10: (61) r1 = *(u32 *)(r7 +24288)
	 R0_w=map_value(id=0,off=0,ks=4,vs=24304,imm=0) R6_w=ctx(id=0,off=0,imm=0) R7_w=map_value(id=0,off=0,ks=4,vs=24304,imm=0) R10=fp0 fp-32=mmmm????
	11: (63) *(u32 *)(r10 -24) = r1
	; struct task_struct *task = (struct task_struct *)get_current_task();
	12: (85) call bpf_get_current_task#35
	13: (bf) r9 = r0
	; struct task_struct *task = (struct task_struct *)get_current_task();
	14: (7b) *(u64 *)(r10 -16) = r9
	; __u32 pid = get_current_pid_tgid() >> 32;
	15: (85) call bpf_get_current_pid_tgid#14
	; __u32 pid = get_current_pid_tgid() >> 32;
...
	regs=4 stack=0 before 1895: (4f) r2 |= r6
	regs=44 stack=0 before 1894: (4f) r2 |= r1
	regs=46 stack=0 before 1893: (67) r2 <<= 24
	regs=46 stack=0 before 1892: (79) r2 = *(u64 *)(r10 -40)
	regs=42 stack=10 before 1891: (67) r1 <<= 16
	regs=42 stack=10 before 1890: (79) r1 = *(u64 *)(r10 -48)
	regs=40 stack=30 before 1889: (4f) r6 |= r1
	regs=42 stack=30 before 1888: (79) r1 = *(u64 *)(r10 -56)
	regs=40 stack=70 before 1887: (67) r6 <<= 8
	regs=40 stack=70 before 1886: (4f) r9 |= r7
	regs=40 stack=70 before 1885: (4f) r9 |= r8
	regs=40 stack=70 before 1884: (67) r9 <<= 24
	regs=40 stack=70 before 1883: (67) r8 <<= 16
	regs=40 stack=70 before 1882: (4f) r7 |= r1
	regs=40 stack=70 before 1881: (79) r1 = *(u64 *)(r10 -64)
	regs=40 stack=70 before 1880: (67) r7 <<= 8
	regs=40 stack=70 before 1879: (15) if r0 == 0x0 goto pc+95
	regs=40 stack=70 before 1878: (85) call bpf_map_lookup_elem#1
	regs=40 stack=70 before 1876: (18) r1 = 0xffff954177317000
	regs=40 stack=70 before 1875: (07) r2 += -16
	regs=40 stack=70 before 1874: (bf) r2 = r10
	regs=40 stack=70 before 1873: (63) *(u32 *)(r10 -16) = r0
	regs=40 stack=70 before 1872: (77) r0 >>= 32
	 R0_rw=inv(id=0) R6_rw=invP(id=0,umax_value=255,var_off=(0x0; 0xff)) R7_rw=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R8_rw=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R9_rw=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R10=fp0 fp-8=mmmm???? fp-16_r=mmmmmmmm fp-24=mmmmmmmm fp-32=mmmm???? fp-40_rw=mmmmmmmm fp-48_rw=mmmmmmmm fp-56_rw=mmmmmmmm fp-64_rw=mmmmmmmm fp-72_rw=mmmmmmmm fp-80_rw=mmmmmmmm fp-88_rw=mmmmmmmm fp-96_r=map_value fp-104=map_value fp-112=ctx fp-120=mmmmmmmm fp-128=mmmmmmmm fp-136_r=map_value fp-144_rw=mmmmmmmm fp-152_rw=mmmmmmmm fp-160_rw=mmmmmmmm fp-168_rw=mmmmmmmm fp-176_rw=mmmmmmmm fp-184_rw=mmmmmmmm fp-192_rw=mmmmmmmm fp-200_rw=mmmmmmmm fp-208_rw=mmmmmmmm
	parent didn't have regs=40 stack=0 marks
	last_idx 1871 first_idx 1821
	regs=40 stack=0 before 1871: (85) call bpf_get_current_pid_tgid#14
	regs=40 stack=0 before 1870: (71) r6 = *(u8 *)(r1 +1)
	math between map_value pointer and register with unbounded min value is not allowed
	processed 3841 insns (limit 1000000) max_states_per_insn 4 total_states 173 peak_states 170 mark_read 136
    logcapture.go:25: time="2024-01-10T10:30:34Z" level=info msg="Unloading sensor __killer__"
    logcapture.go:25: time="2024-01-10T10:30:34Z" level=info msg="BPF prog was unloaded" label=kprobe/killer pin=killer-sensor-1-killer_kprobe
    logcapture.go:25: time="2024-01-10T10:30:34Z" level=info msg="map was unloaded" map=killer_data pin=killer_data
    logcapture.go:25: time="2024-01-10T10:30:34Z" level=info msg="map was unloaded" map=killer_data pin=killer_data
    logcapture.go:25: time="2024-01-10T10:30:34Z" level=info msg="Cleaning up killer"
    observer_test_helper.go:443: SensorManager.AddTracingPolicy error: sensor gtp-sensor-2 from collection killer-override failed to load: failed prog /home/kkourt/src/tetragon/bpf/objs/bpf_generic_tracepoint_v53.o kern_version 328959 loadInstance: opening collection '/home/kkourt/src/tetragon/bpf/objs/bpf_generic_tracepoint_v53.o' failed: program generic_tracepoint_filter: load program: invalid argument: math between map_value pointer and register with unbounded min value is not allowed (18925 line(s) omitted)
    filenames.go:47: keeping export file for TestKillerOverride-override_helper-kprobe_(no_multi) (/tmp/tetragon.gotest.TestKillerOverride-override_helper-kprobe_(no_multi).4012676103.json)
```

It's not clear to me what the issue exatly is, but reading the code I
found an opportunity to simplify it which seems to fix the issue.

Signed-off-by: Kornilios Kourtis <kornilios@isovalent.com>

bpf/lib/bpf_task.h

@@ -155,12 +155,13 @@ static inline __attribute__((always_inline)) struct execve_map_value *
 event_find_curr(__u32 *ppid, bool *walked)
 {
 	struct task_struct *task = (struct task_struct *)get_current_task();
-	__u32 pid = get_current_pid_tgid() >> 32;
 	struct execve_map_value *value = 0;
 	int i;
+	__u32 pid;
 
 #pragma unroll
 	for (i = 0; i < 4; i++) {
+		probe_read(&pid, sizeof(pid), _(&task->tgid));
 		value = execve_map_get_noinit(pid);
 		if (value && value->key.ktime != 0)
 			break;
@@ -169,7 +170,6 @@ event_find_curr(__u32 *ppid, bool *walked)
 		probe_read(&task, sizeof(task), _(&task->real_parent));
 		if (!task)
 			break;
-		probe_read(&pid, sizeof(pid), _(&task->tgid));
 	}
 	*ppid = pid;
 	return value;