nbd: allow genl access outside init_net

NBD doesn't have much to do with networking, allow users outside
init_net to access the family.

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/20230224021301.1630703-1-kuba@kernel.org


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