genetlink: add a family pointer to struct genl_info

Having family in struct genl_info is quite useful. It cuts
down the number of arguments which need to be passed to
helpers which already take struct genl_info.

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20230814214723.2924989-7-kuba@kernel.org


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