f2fs: flush data when enabling checkpoint back

During checkpoint=disable period, f2fs bypasses all the synchronous IOs such as
sync and fsync. So, when enabling it back, we must flush all of them in order
to keep the data persistent. Otherwise, suddern power-cut right after enabling
checkpoint will cause data loss.

Fixes: 4354994f09 ("f2fs: checkpoint disabling")
Cc: stable@vger.kernel.org
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -1892,6 +1892,9 @@ restore_flag:
 
 static void f2fs_enable_checkpoint(struct f2fs_sb_info *sbi)
 {
+	/* we should flush all the data to keep data consistency */
+	sync_inodes_sb(sbi->sb);
+
 	down_write(&sbi->gc_lock);
 	f2fs_dirty_to_prefree(sbi);