null_blk: use blk_mq_init_queue_data

Use the new blk_mq_init_queue_data instead of open coding the queue
allocation and initialization.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>