genetlink: add struct genl_info to struct genl_dumpit_info

Netlink GET implementations must currently juggle struct genl_info
and struct netlink_callback, depending on whether they were called
from doit or dumpit.

Add genl_info to the dump state and populate the fields.
This way implementations can simply pass struct genl_info around.

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20230814214723.2924989-5-kuba@kernel.org


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