nbd: use blk_mq_alloc_disk and blk_cleanup_disk

mainline inclusion
from mainline-v5.14-rc1
commit 4af5f2e0
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I81XCK

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=4af5f2e0301311f88c420fcfc5f3c8611ade20ac



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Use blk_mq_alloc_disk and blk_cleanup_disk to simplify the gendisk and
request_queue allocation.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Link: https://lore.kernel.org/r/20210602065345.355274-20-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Zhong Jinghua <zhongjinghua@huawei.com>
Signed-off-by: Li Nan <linan122@huawei.com>