fix makeRouteTree allowMTU (#611)

With the previous implementation, we check if route.MTU is greater than zero,
but it will always be because we set it to the default MTU in
parseUnsafeRoutes. This change leaves it as zero in parseUnsafeRoutes so
it can be examined later.

overlay/tun_tester.go

@@ -25,7 +25,7 @@ type TestTun struct {
 }
 
 func newTun(l *logrus.Logger, deviceName string, cidr *net.IPNet, _ int, routes []Route, _ int, _ bool) (*TestTun, error) {
-	routeTree, err := makeRouteTree(routes, false)
+	routeTree, err := makeRouteTree(l, routes, false)
 	if err != nil {
 		return nil, err
 	}