ext4: fix panic when mount failed with parallel flush_stashed_error_work

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I46HJ6
CVE: NA
---------------------------

'commit c92dc856 ("ext4: defer saving error info from atomic
context")' and '2d01ddc8 ("ext4: save error info to sb through
journal if available")' add s_error_work to fix checksum error problem.
But the error path in ext4_fill_super can lead the follow BUG_ON.

Our testcase got follow BUG:
[32031.759805] ------------[ cut here ]------------
[32031.759807] kernel BUG at fs/jbd2/transaction.c:373!
[32031.760075] invalid opcode: 0000 [#1] SMP PTI
[32031.760336] CPU: 5 PID: 1029268 Comm: kworker/5:1 Kdump: loaded
Tainted: G           OE    --------- -  -
4.18.0
...
[32031.766665]  jbd2__journal_start+0xf1/0x1f0 [jbd2]
[32031.766934]  jbd2_journal_start+0x19/0x20 [jbd2]
[32031.767218]  flush_stashed_error_work+0x30/0x90 [ext4]
[32031.767487]  process_one_work+0x195/0x390
[32031.767747]  worker_thread+0x30/0x390
[32031.768007]  ? process_one_work+0x390/0x390
[32031.768265]  kthread+0x10d/0x130
[32031.768521]  ? kthread_flush_work_fn+0x10/0x10
[32031.768778]  ret_from_fork+0x35/0x40

static int start_this_handle(...)
    BUG_ON(journal->j_flags & JBD2_UNMOUNT); <---- Trigger this

For this case, flush_stashed_error_work will try to access journal with
parallel ext4_fill_super try to destroy journal. We need make sure there
is no work before me destroy journal. Fix it by flush error work before
we destroy journal.

Fixes: c92dc856 ("ext4: defer saving error info from atomic context")
Fixes: 2d01ddc8 ("ext4: save error info to sb through journal if available")
Signed-off-by: Nyangerkun <yangerkun@huawei.com>
Reviewed-by: NZhang Yi <yi.zhang@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

fs/ext4/super.c

@@ -4832,6 +4832,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 		sbi->s_ea_block_cache = NULL;
 	}
 	if (sbi->s_journal) {
+		flush_work(&sbi->s_error_work);
 		jbd2_journal_destroy(sbi->s_journal);
 		sbi->s_journal = NULL;
 	}