dirauth: IPv6 sybil detection should not use /64

Offending commit is d07f17f6

Basically, a /64 is a network given to end users that is the minimum routable on the Internet iirc.

If dirauth sybil protection uses that, then all relays on the same network won't be able to join the network. At this moment, moria1 is rejecting 435 relays based on that behavior because at least 3 relays are in the same network and thus all get considered as sybil.

The correct thing to do here I believe is that we should use /128 as in match the address only, not the network. Path selection is using /32 here so it is OK to allow multiple relays from the same network as we do in IPv4, just not in the same path.