md: flush ->event_work before stopping array.

[ Upstream commit ee5d004f ]

The 'event_work' worker used by dm-raid may still be running
when the array is stopped.  This can result in an oops.

So flush the workqueue on which it is run after detaching
and before destroying the device.
Reported-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Cc: stable@vger.kernel.org (2.6.38+ please delay 2 weeks after -final release)
Fixes: 9d09e663 ("dm: raid456 basic support")
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

drivers/md/md.c

@@ -5073,6 +5073,8 @@ EXPORT_SYMBOL_GPL(md_stop_writes);
 static void __md_stop(struct mddev *mddev)
 {
 	mddev->ready = 0;
+	/* Ensure ->event_work is done */
+	flush_workqueue(md_misc_wq);
 	mddev->pers->stop(mddev);
 	if (mddev->pers->sync_request && mddev->to_remove == NULL)
 		mddev->to_remove = &md_redundancy_group;