genetlink: make genl_info->nlhdr const

struct netlink_callback has a const nlh pointer, make the
pointer in struct genl_info const as well, to make copying
between the two easier.

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20230814214723.2924989-3-kuba@kernel.org


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