f2fs: complete checkpoints during remount

Otherwise, pending checkpoints can contribute a race condition to give a
quota warning.

- Thread                      - checkpoint thread
                              add checkpoints to the list
do_remount()
 down_write(&sb->s_umount);
 f2fs_remount()
                              block_operations()
                               down_read_trylock(&sb->s_umount) = 0
 up_write(&sb->s_umount);
                               f2fs_quota_sync()
                                dquot_writeback_dquots()
                                 WARN_ON_ONCE(!rwsem_is_locked(&sb->s_umount));

Or,

do_remount()
 down_write(&sb->s_umount);
 f2fs_remount()
                              create a ckpt thread
                              f2fs_enable_checkpoint() adds checkpoints
			      wait for f2fs_sync_fs()
                              trigger another pending checkpoint
                               block_operations()
                                down_read_trylock(&sb->s_umount) = 0
 up_write(&sb->s_umount);
                                f2fs_quota_sync()
                                 dquot_writeback_dquots()
                                  WARN_ON_ONCE(!rwsem_is_locked(&sb->s_umount));

Cc: stable@vger.kernel.org
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -2185,6 +2185,9 @@ static void f2fs_enable_checkpoint(struct f2fs_sb_info *sbi)
 	f2fs_up_write(&sbi->gc_lock);
 
 	f2fs_sync_fs(sbi->sb, 1);
+
+	/* Let's ensure there's no pending checkpoint anymore */
+	f2fs_flush_ckpt_thread(sbi);
 }
 
 static int f2fs_remount(struct super_block *sb, int *flags, char *data)
@@ -2350,6 +2353,9 @@ static int f2fs_remount(struct super_block *sb, int *flags, char *data)
 		f2fs_stop_ckpt_thread(sbi);
 		need_restart_ckpt = true;
 	} else {
+		/* Flush if the prevous checkpoint, if exists. */
+		f2fs_flush_ckpt_thread(sbi);
+
 		err = f2fs_start_ckpt_thread(sbi);
 		if (err) {
 			f2fs_err(sbi,