[PATCH] Add blk_start_queueing() helper

CFQ implements this on its own now, but it's really block layer
knowledge. Tells a device queue to start dispatching requests to
the driver, taking care to unplug if needed. Also fixes the issue
where as/cfq will invoke a stopped queue, which we really don't
want.

Signed-off-by: Jens Axboe <axboe@suse.de>

block/as-iosched.c

@@ -1280,8 +1280,7 @@ static void as_work_handler(void *data)
 	unsigned long flags;
 
 	spin_lock_irqsave(q->queue_lock, flags);
-	if (!as_queue_empty(q))
-		q->request_fn(q);
+	blk_start_queueing(q);
 	spin_unlock_irqrestore(q->queue_lock, flags);
 }