n64cart: convert to blk_alloc_disk

Convert the n64cart driver to use the blk_alloc_disk helper 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/20210521055116.1053587-22-hch@lst.de


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