nbd: Use blk_validate_block_size() to validate block size

Use the block layer helper to validate block size instead
of open coding it.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Link: https://lore.kernel.org/r/20211026144015.188-3-xieyongji@bytedance.com


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