quota: flush quota_release_work upon quota writeback

One of the paths quota writeback is called from is:

freeze_super()
  sync_filesystem()
    ext4_sync_fs()
      dquot_writeback_dquots()

Since we currently don't always flush the quota_release_work queue in
this path, we can end up with the following race:

 1. dquot are added to releasing_dquots list during regular operations.
 2. FS Freeze starts, however, this does not flush the quota_release_work queue.
 3. Freeze completes.
 4. Kernel eventually tries to flush the workqueue while FS is frozen which
    hits a WARN_ON since transaction gets started during frozen state:

  ext4_journal_check_start+0x28/0x110 [ext4] (unreliable)
  __ext4_journal_start_sb+0x64/0x1c0 [ext4]
  ext4_release_dquot+0x90/0x1d0 [ext4]
  quota_release_workfn+0x43c/0x4d0

Which is the following line:

  WARN_ON(sb->s_writers.frozen == SB_FREEZE_COMPLETE);

Which ultimately results in generic/390 failing due to dmesg
noise. This was detected on powerpc machine 15 cores.

To avoid this, make sure to flush the workqueue during
dquot_writeback_dquots() so we dont have any pending workitems after
freeze.

Reported-by: Disha Goel <[email protected]>
CC: [email protected]
Fixes: dabc8b2 ("quota: fix dqput() to follow the guarantees dquot_srcu should provide")
Reviewed-by: Baokun Li <[email protected]>
Signed-off-by: Ojaswin Mujoo <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
Link: https://patch.msgid.link/[email protected]

Author: OjaswinM

fs/quota/dquot.c

@@ -688,6 +688,8 @@ int dquot_writeback_dquots(struct super_block *sb, int type)
 
 	WARN_ON_ONCE(!rwsem_is_locked(&sb->s_umount));
 
+	flush_delayed_work(&quota_release_work);
+
 	for (cnt = 0; cnt < MAXQUOTAS; cnt++) {
 		if (type != -1 && cnt != type)
 			continue;