ext4: use unlikely to improve the efficiency of the kernel

Because the function 'sb_getblk' seldomly fails to return NULL
value,it will be better to use 'unlikely' to optimize it.

Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
index 44734f1..f9b5515 100644
--- a/fs/ext4/mmp.c
+++ b/fs/ext4/mmp.c
@@ -93,7 +93,7 @@
 			*bh = NULL;
 		}
 	}
-	if (!*bh) {
+	if (unlikely(!*bh)) {
 		ext4_warning(sb, "Error while reading MMP block %llu",
 			     mmp_block);
 		return -EIO;