f2fs: fix missing unlock(sbi->gc_mutex)

This fixes missing unlock call.

Cc: <stable@vger.kernel.org>
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -1457,19 +1457,16 @@ static int f2fs_disable_checkpoint(struct f2fs_sb_info *sbi)
 
 	sbi->sb->s_flags |= SB_ACTIVE;
 
-	mutex_lock(&sbi->gc_mutex);
 	f2fs_update_time(sbi, DISABLE_TIME);
 
 	while (!f2fs_time_over(sbi, DISABLE_TIME)) {
+		mutex_lock(&sbi->gc_mutex);
 		err = f2fs_gc(sbi, true, false, NULL_SEGNO);
 		if (err == -ENODATA)
 			break;
-		if (err && err != -EAGAIN) {
-			mutex_unlock(&sbi->gc_mutex);
+		if (err && err != -EAGAIN)
 			return err;
-		}
 	}
-	mutex_unlock(&sbi->gc_mutex);
 
 	err = sync_filesystem(sbi->sb);
 	if (err)