net/tsaddr: simplify TailscaleServiceIP

netaddr allocated at the time this was written. No longer.

name                    old time/op  new time/op  delta
TailscaleServiceAddr-4  5.46ns ± 4%  1.83ns ± 3%  -66.52%  (p=0.008 n=5+5)

A bunch of the others can probably be simplified too, but this
was the only one with just an IP and not an IPPrefix.

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

net/tsaddr/tsaddr_test.go

@@ -93,3 +93,11 @@ func TestNewContainsIPFunc(t *testing.T) {
 		t.Fatal("bad")
 	}
 }
+
+var sinkIP netaddr.IP
+
+func BenchmarkTailscaleServiceAddr(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		sinkIP = TailscaleServiceIP()
+	}
+}