Revert vendor directory to support old golang (GOPATH mode)

gomodule is still in progress to migrate for now, hence multus
team decide to keep vendor directory to support build without
gomodule.

vendor/github.com/vishvananda/netlink/socket.go

@@ -0,0 +1,27 @@
+package netlink
+
+import "net"
+
+// SocketID identifies a single socket.
+type SocketID struct {
+	SourcePort      uint16
+	DestinationPort uint16
+	Source          net.IP
+	Destination     net.IP
+	Interface       uint32
+	Cookie          [2]uint32
+}
+
+// Socket represents a netlink socket.
+type Socket struct {
+	Family  uint8
+	State   uint8
+	Timer   uint8
+	Retrans uint8
+	ID      SocketID
+	Expires uint32
+	RQueue  uint32
+	WQueue  uint32
+	UID     uint32
+	INode   uint32
+}