genetlink: use attrs from struct genl_info

Since dumps carry struct genl_info now, use the attrs pointer
from genl_info and remove the one in struct genl_dumpit_info.

Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20230814214723.2924989-6-kuba@kernel.org


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