block: fix issue with calling blk_stop_queue() from the request_fn handler

When the queue work handler was converted to delayed work, the
stopping was inadvertently made sync as well. Change this back
to being async stop, using __cancel_delayed_work() instead of
cancel_delayed_work().
Reported-by: Jeremy Fitzhardinge <jeremy@goop.org>
Reported-by: Chris Mason <chris.mason@oracle.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/blk-core.c

@@ -271,7 +271,7 @@ EXPORT_SYMBOL(blk_start_queue);
  **/
 void blk_stop_queue(struct request_queue *q)
 {
-	cancel_delayed_work(&q->delay_work);
+	__cancel_delayed_work(&q->delay_work);
 	queue_flag_set(QUEUE_FLAG_STOPPED, q);
 }
 EXPORT_SYMBOL(blk_stop_queue);