md: flush ->event_work before stopping array.

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")

drivers/md/md.c

@@ -5382,6 +5382,8 @@ static void __md_stop(struct mddev *mddev)
 {
 	struct md_personality *pers = mddev->pers;
 	mddev_detach(mddev);
+	/* Ensure ->event_work is done */
+	flush_workqueue(md_misc_wq);
 	spin_lock(&mddev->lock);
 	mddev->ready = 0;
 	mddev->pers = NULL;