nbd: use the structured req attr check

Use the macro for checking presence of required attributes.
It has the advantage of reporting to the user which attr
was missing in a machine-readable format (extack).

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20230224021301.1630703-2-kuba@kernel.org


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