enable hairpin mode on virtual interface bridge port

This is to support being able to DNAT/MASQ traffic from a container back into itself (dotcloud/docker#4442)

Docker-DCO-1.1-Signed-off-by: Patrick Hemmer <patrick.hemmer@gmail.com> (github: phemmer)

network/veth.go

@@ -39,6 +39,9 @@ func (v *Veth) Create(n *Network, nspid int, networkState *NetworkState) error {
 	if err := SetMtu(name1, n.Mtu); err != nil {
 		return err
 	}
+	if err := SetHairpinMode(name1, true); err != nil {
+		return err
+	}
 	if err := InterfaceUp(name1); err != nil {
 		return err
 	}