resolved: size the mdns announce answer array properly

The array doesn't grow dynamically, hence pick the right size at the
moment of allocation. Let's simply multiply the number of addresses of
this link by 2, as that's how many RRs we maintain for it.

src/resolve/resolved-dns-scope.c

@@ -1077,7 +1077,7 @@ int dns_scope_announce(DnsScope *scope, bool goodbye) {
         if (scope->protocol != DNS_PROTOCOL_MDNS)
                 return 0;
 
-        answer = dns_answer_new(4);
+        answer = dns_answer_new(scope->link->n_addresses * 2);
         if (!answer)
                 return log_oom();