Bind Discovery Using Neutral Network Version (#8359)

* fix it

* terence's review

beacon-chain/p2p/discovery.go

@@ -97,15 +97,11 @@ func (s *Service) createListener(
 	ipAddr net.IP,
 	privKey *ecdsa.PrivateKey,
 ) (*discover.UDPv5, error) {
-	// Listen to all network interfaces
-	// for both ip protocols.
-	var networkVersion string
 	// BindIP is used to specify the ip
 	// on which we will bind our listener on
 	// by default we will listen to all interfaces.
 	var bindIP net.IP
-	networkVersion = udpVersionFromIP(ipAddr)
-	switch networkVersion {
+	switch udpVersionFromIP(ipAddr) {
 	case "udp4":
 		bindIP = net.IPv4zero
 	case "udp6":
@@ -121,12 +117,14 @@ func (s *Service) createListener(
 			return nil, errors.New("invalid local ip provided")
 		}
 		bindIP = ipAddr
-		networkVersion = udpVersionFromIP(ipAddr)
 	}
 	udpAddr := &net.UDPAddr{
 		IP:   bindIP,
 		Port: int(s.cfg.UDPPort),
 	}
+	// Listen to all network interfaces
+	// for both ip protocols.
+	networkVersion := "udp"
 	conn, err := net.ListenUDP(networkVersion, udpAddr)
 	if err != nil {
 		return nil, errors.Wrap(err, "could not listen to UDP")