libkern: avoid local var in order_base_2()

order_base_2(n) is implemented with a variable, which keeps it from
being used at file scope. Implement it instead as ilog2(2*n-1), which
produces a different result when 2*n overflows, which appears unlikely
in practice.

Reviewed by:	bz
Differential Revision:	https://reviews.freebsd.org/D46826

(cherry picked from commit b7cbf741d55468ba34305a14ac3acc1c286af034)

sys/sys/libkern.h

@@ -229,10 +229,7 @@ ilog2_long_long(long long n)
 
 #define	ilog2(n) (__builtin_constant_p(n) ? ilog2_const(n) : ilog2_var(n))
 #define	rounddown_pow_of_two(n)	((__typeof(n))1 << ilog2(n))
-#define	order_base_2(n) ({			\
-	__typeof(n) _n = (n);			\
-	_n == 1 ? 0 : 1 + ilog2(_n - 1);	\
-})
+#define order_base_2(n) ilog2(2*(n)-1)
 #define	roundup_pow_of_two(n)	((__typeof(n))1 << order_base_2(n))
 
 #define	bitcount64(x)	__bitcount64((uint64_t)(x))