linux: Avoid risky code swaps with memfd regions

As they may not be writable, and unlike regular regions, ptrace() won't
help us in case of a missing writable bit.

src/linux/frida-helper-backend.vala

@@ -2290,6 +2290,9 @@ namespace Frida {
 			this.session = session;
 
 			Gum.Linux.enumerate_ranges ((Posix.pid_t) session.pid, READ | EXECUTE, d => {
+				unowned Gum.FileMapping? file = d.file;
+				if (file != null && file.path.has_prefix ("memfd:"))
+					return true;
 				if (d.range.size >= code.length) {
 					code_start = d.range.base_address + d.range.size - round_size_to_page_size (code.length);
 					code_end = code_start + code.length;