network: fix SIGABRT related to unreachable route with DHCP6

After #17834, unreachable routes generated through DHCP6 are managed by
Manager. But they are referrenced by the DHCP6 uplink. So, the routes
managed by Manager must be freed after all Link objects are freed.

Follow-up for 575f14eef0.

Fixes SIGABRT reproted in #17831.

src/network/networkd-manager.c

@@ -888,13 +888,17 @@ void manager_free(Manager *m) {
         m->rules_foreign = set_free(m->rules_foreign);
         set_free(m->rules_saved);
 
-        m->routes = set_free(m->routes);
-        m->routes_foreign = set_free(m->routes_foreign);
-
         sd_netlink_unref(m->rtnl);
         sd_netlink_unref(m->genl);
         sd_resolve_unref(m->resolve);
 
+        /* reject (e.g. unreachable) type routes are managed by Manager, but may be referenced by a
+         * link. E.g., DHCP6 with prefix delegation creates unreachable routes, and they are referenced
+         * by the upstream link. And the links may be referenced by netlink slots. Hence, two
+         * set_free() must be called after the above sd_netlink_unref(). */
+        m->routes = set_free(m->routes);
+        m->routes_foreign = set_free(m->routes_foreign);
+
         sd_event_source_unref(m->speed_meter_event_source);
         sd_event_unref(m->event);