platform: get extack_msg innm_platform_ip_route_sync()

Request the extack_msg for nm_platform_ip_route_add() call. Note that we (currently)
don't do anything with it, however requesting it has no downsides. That is, the
message already is heap allocated in the lower layers, so this only affects whether
it will be returned up to nm_platform_ip_route_sync().

src/libnm-platform/nm-platform.c

@@ -4928,8 +4928,10 @@ nm_platform_ip_route_sync(NMPlatform *self,
 
     for (i_type = 0; routes && i_type < 2; i_type++) {
         for (i = 0; i < routes->len; i++) {
-            int      r, r2;
+            gs_free char *extack_msg          = NULL;
             gboolean      gateway_route_added = FALSE;
+            int           r2;
+            int           r;
 
             conf_o = routes->pdata[i];
 
@@ -4990,7 +4992,7 @@ sync_route_add:
                                          NMP_NLM_FLAG_APPEND
                                              | NMP_NLM_FLAG_SUPPRESS_NETLINK_FAILURE,
                                          conf_o,
-                                         NULL);
+                                         &extack_msg);
             if (r < 0) {
                 if (r == -EEXIST) {
                     /* Don't fail for EEXIST. It's not clear that the existing route