plugins/loopback: take lo down on CmdDel

plugins/main/loopback/loopback.go

@@ -33,7 +33,24 @@ func cmdAdd(args *skel.CmdArgs) error {
 }
 
 func cmdDel(args *skel.CmdArgs) error {
-	// del does nothing, we're going to destroy the device anyway
+	args.IfName = "lo" // ignore config, this only works for loopback
+	err := ns.WithNetNSPath(args.Netns, false, func(hostNS *os.File) error {
+		link, err := netlink.LinkByName(args.IfName)
+		if err != nil {
+			return err // not tested
+		}
+
+		err = netlink.LinkSetDown(link)
+		if err != nil {
+			return err // not tested
+		}
+
+		return nil
+	})
+	if err != nil {
+		return err // not tested
+	}
+
 	return nil
 }