Fix tunnelDstIP field in traceflow on dual-stack

On dual-stack testbed, Inter-Node traceflow observation for
IPv4 packet in encap mode has empty tunnelDstIP field because
isIPv6 variable is set.

Change the logic to set isIPv6 variable based on the packet
IPv4 or IPv6.

Fixes antrea-io#4502

Signed-off-by: Kumar Atish <atish.iaf@gmail.com>

pkg/agent/controller/traceflow/packetin.go

@@ -237,7 +237,8 @@ func (c *Controller) parsePacketIn(pktIn *ofctrl.PacketIn) (*crdv1alpha1.Tracefl
 	if tableID == openflow.L2ForwardingOutTable.GetID() {
 		ob := new(crdv1alpha1.Observation)
 		tunnelDstIP := ""
-		isIPv6 := c.nodeConfig.NodeIPv6Addr != nil
+		// decide according to packet.
+		isIPv6 := etherData.Ethertype == protocol.IPv6_MSG
 		if match := getMatchTunnelDstField(matchers, isIPv6); match != nil {
 			tunnelDstIP, err = getTunnelDstValue(match)
 			if err != nil {