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/xfrm_monitor_linux.go

@@ -0,0 +1,98 @@
+package netlink
+
+import (
+	"fmt"
+	"syscall"
+
+	"github.com/vishvananda/netns"
+
+	"github.com/vishvananda/netlink/nl"
+)
+
+type XfrmMsg interface {
+	Type() nl.XfrmMsgType
+}
+
+type XfrmMsgExpire struct {
+	XfrmState *XfrmState
+	Hard      bool
+}
+
+func (ue *XfrmMsgExpire) Type() nl.XfrmMsgType {
+	return nl.XFRM_MSG_EXPIRE
+}
+
+func parseXfrmMsgExpire(b []byte) *XfrmMsgExpire {
+	var e XfrmMsgExpire
+
+	msg := nl.DeserializeXfrmUserExpire(b)
+	e.XfrmState = xfrmStateFromXfrmUsersaInfo(&msg.XfrmUsersaInfo)
+	e.Hard = msg.Hard == 1
+
+	return &e
+}
+
+func XfrmMonitor(ch chan<- XfrmMsg, done <-chan struct{}, errorChan chan<- error,
+	types ...nl.XfrmMsgType) error {
+
+	groups, err := xfrmMcastGroups(types)
+	if err != nil {
+		return nil
+	}
+	s, err := nl.SubscribeAt(netns.None(), netns.None(), syscall.NETLINK_XFRM, groups...)
+	if err != nil {
+		return err
+	}
+
+	if done != nil {
+		go func() {
+			<-done
+			s.Close()
+		}()
+
+	}
+
+	go func() {
+		defer close(ch)
+		for {
+			msgs, err := s.Receive()
+			if err != nil {
+				errorChan <- err
+				return
+			}
+			for _, m := range msgs {
+				switch m.Header.Type {
+				case nl.XFRM_MSG_EXPIRE:
+					ch <- parseXfrmMsgExpire(m.Data)
+				default:
+					errorChan <- fmt.Errorf("unsupported msg type: %x", m.Header.Type)
+				}
+			}
+		}
+	}()
+
+	return nil
+}
+
+func xfrmMcastGroups(types []nl.XfrmMsgType) ([]uint, error) {
+	groups := make([]uint, 0)
+
+	if len(types) == 0 {
+		return nil, fmt.Errorf("no xfrm msg type specified")
+	}
+
+	for _, t := range types {
+		var group uint
+
+		switch t {
+		case nl.XFRM_MSG_EXPIRE:
+			group = nl.XFRMNLGRP_EXPIRE
+		default:
+			return nil, fmt.Errorf("unsupported group: %x", t)
+		}
+
+		groups = append(groups, group)
+	}
+
+	return groups, nil
+}