nbd: validate the block size in nbd_set_size

mainline inclusion
from mainline-v5.1-rc1
commit dcbddf54
category: bugfix
bugzilla: 188268
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=dcbddf541f18e367ac9cdad8e223d382cd303161



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Move the validation of the block from the callers into nbd_set_size.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Zhong Jinghua <zhongjinghua@huawei.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>