brd: convert to blk_alloc_disk/blk_cleanup_disk

Convert the brd driver to use the blk_alloc_disk and blk_cleanup_disk
helpers to simplify gendisk and request_queue allocation.  This also
allows to remove the request_queue pointer in struct request_queue,
and to simplify the initialization as blk_cleanup_disk can be called
on any disk returned from blk_alloc_disk.

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/20210521055116.1053587-7-hch@lst.de


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