Add IFLA_INET6_ADDR_GEN_MODE accessors

[vifino]

See the commit to libnl that adds such support.

Allows setting IN6_ADDR_GEN_MODE_NONE, IN6_ADDR_GEN_MODE_EUI64 or IN6_ADDR_GEN_MODE_STABLE_PRIVACY.

Unfortunately, I haven't quite figured out how to add support for this myself.
If I do, I'll PR.