wgengine/filter: drop multicast packets out, don't log about them

Eventually we'll probably support multicast. For now it's just log spam.

Fixes #629

wgengine/filter/filter.go

@@ -367,6 +367,11 @@ func (f *Filter) pre(q *packet.ParsedPacket, rf RunFlags, dir direction) Respons
 		f.logRateLimit(rf, q, dir, Drop, "ipv6")
 		return Drop
 	}
+	if q.DstIP.IsMulticast() {
+		f.logRateLimit(rf, q, dir, Drop, "multicast")
+		return Drop
+	}
+
 	switch q.IPProto {
 	case packet.Unknown:
 		// Unknown packets are dangerous; always drop them.
@@ -409,6 +414,9 @@ func omitDropLogging(p *packet.ParsedPacket, dir direction) bool {
 			if ipProto == packet.IGMP {
 				return true
 			}
+			if p.DstIP.IsMulticast() {
+				return true
+			}
 		case 6:
 			if len(b) < 40 {
 				return false