networkd: Assign VXLAN destination port to when GPE is set

When VXLAN destination port is unset and GPE is set
then assign 4790 to destination port. Kernel does the same as
well as iproute.

IANA VXLAN-GPE port is 4790

src/network/netdev/vxlan.c

@@ -289,6 +289,9 @@ static int netdev_vxlan_verify(NetDev *netdev, const char *filename) {
                                                 "%s: VXLAN TTL must be <= 255. Ignoring.",
                                                 filename);
 
+        if (!v->dest_port && v->generic_protocol_extension)
+                v->dest_port = 4790;
+
         return 0;
 }