fs: Move bh_cachep to the __read_mostly section

bh_cachep is only written to once on initialization, so move it to the
__read_mostly section.
Signed-off-by: Shai Fultheim <shai@scalemp.com>
Signed-off-by: Vlad Zolotarov <vlad@scalemp.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/buffer.c

@@ -3152,7 +3152,7 @@ SYSCALL_DEFINE2(bdflush, int, func, long, data)
 /*
  * Buffer-head allocation
  */
-static struct kmem_cache *bh_cachep;
+static struct kmem_cache *bh_cachep __read_mostly;
 
 /*
  * Once the number of bh's in the machine exceeds this level, we start