net/interfaces: get Linux default route from netlink as fallback

If it's in a non-standard table, as it is on Unifi UDM Pro, apparently.

Updates #4038 (probably fixes, but don't have hardware to verify)

Change-Id: I2cb9a098d8bb07d1a97a6045b686aca31763a937
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/interfaces/interfaces_linux_test.go

@@ -5,7 +5,9 @@
 package interfaces
 
 import (
+	"errors"
 	"fmt"
+	"io/fs"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -107,3 +109,14 @@ func BenchmarkDefaultRouteInterface(b *testing.B) {
 		}
 	}
 }
+
+func TestRouteLinuxNetlink(t *testing.T) {
+	d, err := defaultRouteFromNetlink()
+	if errors.Is(err, fs.ErrPermission) {
+		t.Skip(err)
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("Got: %+v", d)
+}