genetlink: add genlmsg_iput() API

Add some APIs and helpers required for convenient construction
of replies and notifications based on struct genl_info.

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20230814214723.2924989-8-kuba@kernel.org


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