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/nl/mpls_linux.go

@@ -0,0 +1,36 @@
+package nl
+
+import "encoding/binary"
+
+const (
+	MPLS_LS_LABEL_SHIFT = 12
+	MPLS_LS_S_SHIFT     = 8
+)
+
+func EncodeMPLSStack(labels ...int) []byte {
+	b := make([]byte, 4*len(labels))
+	for idx, label := range labels {
+		l := label << MPLS_LS_LABEL_SHIFT
+		if idx == len(labels)-1 {
+			l |= 1 << MPLS_LS_S_SHIFT
+		}
+		binary.BigEndian.PutUint32(b[idx*4:], uint32(l))
+	}
+	return b
+}
+
+func DecodeMPLSStack(buf []byte) []int {
+	if len(buf)%4 != 0 {
+		return nil
+	}
+	stack := make([]int, 0, len(buf)/4)
+	for len(buf) > 0 {
+		l := binary.BigEndian.Uint32(buf[:4])
+		buf = buf[4:]
+		stack = append(stack, int(l)>>MPLS_LS_LABEL_SHIFT)
+		if (l>>MPLS_LS_S_SHIFT)&1 > 0 {
+			break
+		}
+	}
+	return stack
+}