networkd: set IFLA_INET6_ADDR_GEN_MODE as per stable_secret

networkd: set IFLA_INET6_ADDR_GEN_MODE as per stable_secret

Although networkd has option (LinkLocalAddressing=) to toggle IPv6LL autoconfiguration, when it is enabled, the address is autoconfigured by the kernel, but not networkd.

Therefore, we do not statically set IFLA_INET6_ADDR_GEN_MODE to IN6_ADDR_GEN_MODE_EUI64, but dynamically depending on whether stable_secret is set, just as what the kernel does by default.

Note that this does NOT affect the global addresses configured by networkd.