netdev-genl: use struct genl_info for reply construction

Use the just added APIs to make the code simpler.

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20230814214723.2924989-9-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>