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>
diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
index 2886aef..29ceb9d 100644
--- a/fs/f2fs/gc.c
+++ b/fs/f2fs/gc.c
@@ -119,7 +119,6 @@
 		kfree(gc_th);
 		sbi->gc_thread = NULL;
 	}
-
 out:
 	return err;
 }
@@ -695,7 +694,7 @@
 
 	INIT_LIST_HEAD(&ilist);
 gc_more:
-	if (!(sbi->sb->s_flags & MS_ACTIVE))
+	if (unlikely(!(sbi->sb->s_flags & MS_ACTIVE)))
 		goto stop;
 
 	if (gc_type == BG_GC && has_not_enough_free_secs(sbi, nfree)) {