nvdimm-btt: convert to blk_alloc_disk/blk_cleanup_disk

Convert the nvdimm-btt driver to use the blk_alloc_disk and
blk_cleanup_disk helpers to simplify gendisk and request_queue
allocation.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20210521055116.1053587-17-hch@lst.de


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