journal: do not accept multiple requests to sync journal per varlink connection

Otherwise, previously allocated SyncReq object will be leaked,
and the latest SyncReq object will be freed multiple times.

src/journal/journald-varlink.c

@@ -40,11 +40,17 @@ static int vl_method_synchronize(sd_varlink *link, sd_json_variant *parameters,
                 {}
         };
 
-        Manager *m = ASSERT_PTR(userdata);
         int r;
 
         assert(link);
 
+        /* Here, userdata may not be a pointer to the Manager object. Obtain the manager from the server. */
+        Manager *m = ASSERT_PTR(sd_varlink_server_get_userdata(sd_varlink_get_server(link)));
+
+        /* The varlink connection already requested to sync journals. Refusing. */
+        if (sd_varlink_get_userdata(link) != m)
+                return -EBUSY;
+
         r = sd_varlink_dispatch(link, parameters, dispatch_table, &offline);
         if (r != 0)
                 return r;