feature/linkspeed: move cosmetic tstun netlink code out to modular feature

Part of making all netlink monitoring code optional.

Updates #17311 (how I got started down this path)
Updates #12614

Change-Id: Ic80d8a7a44dc261c4b8678b3c2241c3b3778370d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tstun/tun.go

@@ -18,12 +18,16 @@ import (
 
 	"github.com/tailscale/wireguard-go/tun"
 	"tailscale.com/feature"
+	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/types/logger"
 )
 
-// CreateTAP is the hook set by feature/tap.
+// CreateTAP is the hook maybe set by feature/tap.
 var CreateTAP feature.Hook[func(logf logger.Logf, tapName, bridgeName string) (tun.Device, error)]
 
+// HookSetLinkAttrs is the hook maybe set by feature/linkspeed.
+var HookSetLinkAttrs feature.Hook[func(tun.Device) error]
+
 // modprobeTunHook is a Linux-specific hook to run "/sbin/modprobe tun".
 var modprobeTunHook feature.Hook[func() error]
 
@@ -78,8 +82,12 @@ func New(logf logger.Logf, tunName string) (tun.Device, string, error) {
 		dev.Close()
 		return nil, "", err
 	}
-	if err := setLinkAttrs(dev); err != nil {
-		logf("setting link attributes: %v", err)
+	if buildfeatures.HasLinkSpeed {
+		if f, ok := HookSetLinkAttrs.GetOk(); ok {
+			if err := f(dev); err != nil {
+				logf("setting link attributes: %v", err)
+			}
+		}
 	}
 	name, err := interfaceName(dev)
 	if err != nil {