tap: Don't update ip6.addr_seen to ::

When we receive packets from the tap side, we update the addr_seen fields
to reflect the last known address of the guest or ns.  For ip4.addr_seen
we, sensibly, only update if the address we've just seen isn't 0 (0.0.0.0).
This case can occur during early DHCP transactions.

We have no equivalent case for IPv6.  We're less likely to hit this,
because DHCPv6 uses link-local addresses, however we can see an source
address of :: with certain multicast operations.  This can bite us if we
try to make an incoming connection very early after starting pasta with
--config-net: we may have only seen some of those multicast packets,
updated addr_seen to :: and not had any "real" packets to update it to a
global address.  I've seen this with some of the avocado test conversions.

In any case, it can never make sense to update addr_seen to ::, so
explicitly exclude that case.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

tap.c

@@ -739,7 +739,7 @@ resume:
 			if (IN6_IS_ADDR_UNSPECIFIED(&c->ip6.addr_seen)) {
 				c->ip6.addr_seen = *saddr;
 			}
-		} else {
+		} else if (!IN6_IS_ADDR_UNSPECIFIED(saddr)){
 			c->ip6.addr_seen = *saddr;
 		}