netlink: make range pointers in policies const

struct nla_policy is usually constant itself, but unless
we make the ranges inside constant we won't be able to
make range structs const. The ranges are not modified
by the core.

Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20231025162204.132528-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>