Use IANA assigned VXLAN port

Fixes #358

Signed-off-by: Dave Tucker <dt@docker.com>

drivers/overlay/ov_utils.go

@@ -54,6 +54,7 @@ func createVxlan(vni uint32) (string, error) {
 		LinkAttrs: netlink.LinkAttrs{Name: name},
 		VxlanId:   int(vni),
 		Learning:  true,
+		Port:      vxlanPort,
 		Proxy:     true,
 		L3miss:    true,
 		L2miss:    true,

drivers/overlay/overlay.go

@@ -21,6 +21,7 @@ const (
 	vethLen      = 7
 	vxlanIDStart = 256
 	vxlanIDEnd   = 1000
+	vxlanPort    = 4789
 )
 
 type driver struct {