f2fs: add unlikely() macro for compiler more aggressively

This patch adds unlikely() macro into the most of codes.
The basic rule is to add that when:
- checking unusual errors,
- checking page mappings,
- and the other unlikely conditions.

Change log from v1:
 - Don't add unlikely for the NULL test and error test: advised by Andi Kleen.

Cc: Chao Yu <chao2.yu@samsung.com>
Cc: Andi Kleen <andi@firstfloor.org>
Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/recovery.c

@@ -430,7 +430,7 @@ int recover_fsync_data(struct f2fs_sb_info *sbi)
 
 	fsync_entry_slab = f2fs_kmem_cache_create("f2fs_fsync_inode_entry",
 			sizeof(struct fsync_inode_entry), NULL);
-	if (unlikely(!fsync_entry_slab))
+	if (!fsync_entry_slab)
 		return -ENOMEM;
 
 	INIT_LIST_HEAD(&inode_list);