blk-mq: stop hardware queue in blk_mq_delay_queue()

The run handler we register for the delayed work requires that the
queue be stopped, yet we leave that up to the caller. Let's move
it into blk_mq_delay_queue() itself, so that the API is sane.

This fixes a stall with SCSI, where it calls blk_mq_delay_queue()
without having stopped the queue. Hence the queue is never run.

Reported-by: Hannes Reinecke <hare@suse.com>
Fixes: 70f4db63

 ("blk-mq: add blk_mq_delay_queue")
Signed-off-by: Jens Axboe <axboe@fb.com>