nbd: Remove superfluous casts

In Linux kernel code it is preferred not to use a cast when converting a
void pointer to another pointer type.

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/20240510202313.25209-3-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>