fs/mbcache: replace __builtin_log2() with ilog2()

Fix compiler error with some gcc version(s) that do not
support __builtin_log2() by replacing __builtin_log2() with
ilog2().
Signed-off-by: NT. Makphaibulchoke <tmac@hp.com>
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>
Reviewed-by: NMaciej W. Rozycki <macro@linux-mips.org>

fs/mbcache.c

@@ -73,6 +73,7 @@
 #include <linux/mbcache.h>
 #include <linux/init.h>
 #include <linux/blockgroup_lock.h>
+#include <linux/log2.h>
 
 #ifdef MB_CACHE_DEBUG
 # define mb_debug(f...) do { \
@@ -93,7 +94,7 @@
 
 #define MB_CACHE_WRITER ((unsigned short)~0U >> 1)
 
-#define MB_CACHE_ENTRY_LOCK_BITS	__builtin_log2(NR_BG_LOCKS)
+#define MB_CACHE_ENTRY_LOCK_BITS	ilog2(NR_BG_LOCKS)
 #define	MB_CACHE_ENTRY_LOCK_INDEX(ce)			\
 	(hash_long((unsigned long)ce, MB_CACHE_ENTRY_LOCK_BITS))