net/netns: optimize defaultRouteInterface a bit

It'll be called a bunch, so worth a bit of effort. Could go further, but not yet.
(really, should hook into wgengine/monitor and only re-read on netlink changes?)

name                     old time/op    new time/op    delta
DefaultRouteInterface-8    60.8µs ±11%    44.6µs ± 5%  -26.65%  (p=0.000 n=20+19)

name                     old alloc/op   new alloc/op   delta
DefaultRouteInterface-8    3.29kB ± 0%    0.55kB ± 0%  -83.21%  (p=0.000 n=20+20)

name                     old allocs/op  new allocs/op  delta
DefaultRouteInterface-8      9.00 ± 0%      6.00 ± 0%  -33.33%  (p=0.000 n=20+20)

net/netns/netns_linux_test.go

@@ -49,3 +49,12 @@ func TestBypassMarkInSync(t *testing.T) {
 	}
 	t.Errorf("tailscaleBypassMark not found in router_linux.go")
 }
+
+func BenchmarkDefaultRouteInterface(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		if _, err := defaultRouteInterface(); err != nil {
+			b.Fatal(err)
+		}
+	}
+}