blk-mq: fix deadlock in blk_mq_register_disk() error path

If we fail registering any of the hardware queues, we call
into blk_mq_unregister_disk() with the hotplug mutex already
held. Since blk_mq_unregister_disk() attempts to acquire the
same mutex, we end up in a less than happy place.

Reported-by: Jinpu Wang <jinpu.wang@profitbricks.com>
Signed-off-by: Jens Axboe <axboe@fb.com>