Change `AfSpecInet6::AddrGenMode(u8)` to enum.

Currently, there is no way to use any of the values of [`in6_addr_gen_mod`](https://github.com/torvalds/linux/blob/05dbaf8dd8bf537d4b4eb3115ab42a5fb40ff1f5/include/uapi/linux/if_link.h#L456) although [`netlink_packet_route::link::AfSpecInet6::AddrGenMode`](https://docs.rs/netlink-packet-route/latest/netlink_packet_route/link/enum.AfSpecInet6.html) is already exposed.