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>

cmd/tailscaled/deps_test.go

@@ -285,8 +285,9 @@ func TestMinTailscaledWithCLI(t *testing.T) {
 			}
 		},
 		BadDeps: map[string]string{
-			"golang.org/x/net/http2": "unexpected x/net/http2 dep; tailscale/tailscale#17305",
-			"expvar":                 "unexpected expvar dep",
+			"golang.org/x/net/http2":        "unexpected x/net/http2 dep; tailscale/tailscale#17305",
+			"expvar":                        "unexpected expvar dep",
+			"github.com/mdlayher/genetlink": "unexpected genetlink dep",
 		},
 	}.Check(t)
 }