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/link_tuntap_linux.go

@@ -0,0 +1,14 @@
+package netlink
+
+// ideally golang.org/x/sys/unix would define IfReq but it only has
+// IFNAMSIZ, hence this minimalistic implementation
+const (
+	SizeOfIfReq = 40
+	IFNAMSIZ    = 16
+)
+
+type ifReq struct {
+	Name  [IFNAMSIZ]byte
+	Flags uint16
+	pad   [SizeOfIfReq - IFNAMSIZ - 2]byte
+}