nbd: Remove a local variable from nbd_send_cmd()

blk_rq_bytes() returns an unsigned int while 'size' has type unsigned long.
This is confusing. Improve code readability by removing the local variable
'size'.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Josef Bacik <jbacik@fb.com>
Cc: Yu Kuai <yukuai3@huawei.com>
Cc: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20240510202313.25209-5-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>