Handle empty netns in DEL for loopback and host-device.

Signed-off-by: Lantao Liu <lantaol@google.com>

Author: Random-Liu

plugins/main/host-device/host-device.go

@@ -85,6 +85,9 @@ func cmdDel(args *skel.CmdArgs) error {
 	if err != nil {
 		return err
 	}
+	if args.Netns == "" {
+		return nil
+	}
 	containerNs, err := ns.GetNS(args.Netns)
 	if err != nil {
 		return fmt.Errorf("failed to open netns %q: %v", args.Netns, err)

plugins/main/loopback/loopback.go

@@ -48,6 +48,9 @@ func cmdAdd(args *skel.CmdArgs) error {
 }
 
 func cmdDel(args *skel.CmdArgs) error {
+	if args.Netns == "" {
+		return nil
+	}
 	args.IfName = "lo" // ignore config, this only works for loopback
 	err := ns.WithNetNSPath(args.Netns, func(ns.NetNS) error {
 		link, err := netlink.LinkByName(args.IfName)