nvme: use blk_mq_alloc_disk

Switch to use the blk_mq_alloc_disk helper for allocating the
request_queue and gendisk.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Link: https://lore.kernel.org/r/20210816131910.615153-2-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>