dm: run queue on re-queue

Without kicking queue, requeued request may stay forever in
the queue if there are no other I/O activities to the device.

The original error had been in v2.6.39 with commit 7eaceacc
("block: remove per-queue plugging"), which replaced conditional
plugging by periodic runqueue.

Commit 9d1deb83 in v4.1-rc1 removed the periodic runqueue
and the problem started to manifest.

Fixes: 9d1deb83 ("dm: don't schedule delayed run of the queue if nothing to do")
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm.c

@@ -1164,6 +1164,7 @@ static void old_requeue_request(struct request *rq)
 
 	spin_lock_irqsave(q->queue_lock, flags);
 	blk_requeue_request(q, rq);
+	blk_run_queue_async(q);
 	spin_unlock_irqrestore(q->queue_lock, flags);
 }