iface-helper: fix RA processing with more than one router

iface-helper: fix RA processing with more than one router

We construct new IP6Config on each rdisc_config_changed(). That's not a smart
thing to do, since that makes us throw away the previous configuration.

In case the two routers on the same network, the first RA triggers
rdisc_config_changed() for changed gateway and addresses. On handling the
second RA rdisc_config_changed() doesn't add the address, resulting in the
address being removed on ip6 config sync.

A side effect of this is that the address is still tentative, resulting in DAD
retry and an new address being added. So the networking still works, but at the
expense of a single DAD failure and an address that's perhaps different from what
was expected.