nixos/networking: fix v4+v6 default gateways with networkd

fixes #178078

nixos/modules/tasks/network-interfaces-systemd.nix

@@ -59,15 +59,14 @@ in
         genericNetwork = override:
           let gateway = optional (cfg.defaultGateway != null && (cfg.defaultGateway.address or "") != "") cfg.defaultGateway.address
             ++ optional (cfg.defaultGateway6 != null && (cfg.defaultGateway6.address or "") != "") cfg.defaultGateway6.address;
-          in optionalAttrs (gateway != [ ]) {
-            routes = override [
-              {
+              makeGateway = gateway: {
                 routeConfig = {
                   Gateway = gateway;
                   GatewayOnLink = false;
                 };
-              }
-            ];
+              };
+          in optionalAttrs (gateway != [ ]) {
+            routes = override (map makeGateway gateway);
           } // optionalAttrs (domains != [ ]) {
             domains = override domains;
           };

nixos/tests/networking.nix

@@ -98,6 +98,7 @@ let
           useNetworkd = networkd;
           useDHCP = false;
           defaultGateway = "192.168.1.1";
+          defaultGateway6 = "fd00:1234:5678:1::1";
           interfaces.eth1.ipv4.addresses = mkOverride 0 [
             { address = "192.168.1.2"; prefixLength = 24; }
             { address = "192.168.1.3"; prefixLength = 32; }
@@ -139,6 +140,8 @@ let
           with subtest("Test default gateway"):
               router.wait_until_succeeds("ping -c 1 192.168.3.1")
               client.wait_until_succeeds("ping -c 1 192.168.3.1")
+              router.wait_until_succeeds("ping -c 1 fd00:1234:5678:3::1")
+              client.wait_until_succeeds("ping -c 1 fd00:1234:5678:3::1")
         '';
     };
     routeType = {