scoutfs-utils: add ilog2() helper

It's handy to use ilog2 in the format header for defining shifts based
on values.  Add a userspace helper that uses glibc's log2 functions.

Signed-off-by: Zach Brown <zab@versity.com>

utils/src/util.h

@@ -4,6 +4,7 @@
 #include <unistd.h>
 #include <stdlib.h>
 #include <string.h>
+#include <math.h>
 
 /*
  * Generate build warnings if the condition is false but generate no
@@ -79,6 +80,11 @@ do {				\
 	(_x == 0 ? 0 : 64 - __builtin_clzll(_x));	\
 })
 
+#define ilog2(x)					\
+({							\
+	((unsigned long)log2l((long double)x));		\
+})
+
 /*
  * return -1,0,+1 based on the memcmp comparison of the minimum of their
  * two lengths.  If their min shared bytes are equal but the lengths