nbd: Remove __force casts

nbd: Remove __force casts

Make it again possible for sparse to verify that blk_status_t and Unix
error codes are used in the proper context by making nbd_send_cmd()
return a blk_status_t instead of an integer.

No functionality has been changed.

Signed-off-by: Christoph Hellwig <hch@lst.de>
[ bvanassche: added description and made two small formatting changes ]
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20240604221531.327131-1-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>