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: NJinpu Wang <jinpu.wang@profitbricks.com>
Signed-off-by: NJens Axboe <axboe@fb.com>