f2fs: avoid unable to restart gc thread in remount

In f2fs_remount, we will stop gc thread and set need_restart_gc as true when new
option is set without BG_GC, then if any error occurred in the following
procedure, we can restore to start the gc thread.
But after that, We will fail to restore gc thread in start_gc_thread as BG_GC is
not set in new option, so we'd better move this condition judgment out of
start_gc_thread to fix this issue.
Signed-off-by: NChao Yu <chao2.yu@samsung.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/gc.c

@@ -96,8 +96,6 @@ int start_gc_thread(struct f2fs_sb_info *sbi)
 	dev_t dev = sbi->sb->s_bdev->bd_dev;
 	int err = 0;
 
-	if (!test_opt(sbi, BG_GC))
-		goto out;
 	gc_th = kmalloc(sizeof(struct f2fs_gc_kthread), GFP_KERNEL);
 	if (!gc_th) {
 		err = -ENOMEM;

fs/f2fs/super.c

@@ -1138,7 +1138,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 	 * If filesystem is not mounted as read-only then
 	 * do start the gc_thread.
 	 */
-	if (!f2fs_readonly(sb)) {
+	if (test_opt(sbi, BG_GC) && !f2fs_readonly(sb)) {
 		/* After POR, we can run background GC thread.*/
 		err = start_gc_thread(sbi);
 		if (err)