f2fs: flush quota blocks after turnning it off

After quota_off, we'll get some dirty blocks. If put_super don't have a chance
to flush them by checkpoint, it causes NULL pointer exception in end_io after
iput(node_inode). (e.g., by checkpoint=disable)
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -2026,6 +2026,12 @@ void f2fs_quota_off_umount(struct super_block *sb)
 			set_sbi_flag(F2FS_SB(sb), SBI_QUOTA_NEED_REPAIR);
 		}
 	}
+	/*
+	 * In case of checkpoint=disable, we must flush quota blocks.
+	 * This can cause NULL exception for node_inode in end_io, since
+	 * put_super already dropped it.
+	 */
+	sync_filesystem(sb);
 }
 
 static void f2fs_truncate_quota_inode_pages(struct super_block *sb)