md: flush ->event_work before stopping array.

commit ee5d004f upstream.

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>
Fixes: 9d09e663 ("dm: raid456 basic support")
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/md/md.c

@@ -5264,6 +5264,8 @@ EXPORT_SYMBOL_GPL(md_stop_writes);
 
 static void __md_stop(struct mddev *mddev)
 {
+	/* Ensure ->event_work is done */
+	flush_workqueue(md_misc_wq);
 	mddev->ready = 0;
 	mddev->pers->stop(mddev);
 	if (mddev->pers->sync_request && mddev->to_remove == NULL)