dm: use blk_set_queue_dying() in __dm_destroy()

After QUEUE_FLAG_DYING has been set any code that is waiting in
get_request() should be woken up.  But to get this behaviour
blk_set_queue_dying() must be used instead of only setting
QUEUE_FLAG_DYING.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm.c

@@ -1886,9 +1886,7 @@ static void __dm_destroy(struct mapped_device *md, bool wait)
 	set_bit(DMF_FREEING, &md->flags);
 	spin_unlock(&_minor_lock);
 
-	spin_lock_irq(q->queue_lock);
-	queue_flag_set(QUEUE_FLAG_DYING, q);
-	spin_unlock_irq(q->queue_lock);
+	blk_set_queue_dying(q);
 
 	if (dm_request_based(md) && md->kworker_task)
 		kthread_flush_worker(&md->kworker);