net/tstun: set link speed to SPEED_UNKNOWN

Fixes #3933.

Signed-off-by: Dmytro Shynkevych <dm.shynk@gmail.com>

net/tstun/linkattrs_notlinux.go

@@ -0,0 +1,14 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !linux
+// +build !linux
+
+package tstun
+
+import "golang.zx2c4.com/wireguard/tun"
+
+func setLinkAttrs(iface tun.Device) error {
+	return nil
+}