cgroup: only check once when mode is UNIT_CHECK

If the mode is UNIT_CHECK,it means we only want to check if
the paramaters are valid. the first round of cycle already
did this check, no need to check again.

src/core/dbus-unit.c

@@ -981,7 +981,7 @@ int bus_unit_set_properties(
 
                 if (dbus_message_iter_get_arg_type(&sub) == DBUS_TYPE_INVALID) {
 
-                        if (for_real)
+                        if (for_real || mode == UNIT_CHECK)
                                 break;
 
                         /* Reached EOF. Let's try again, and this time for realz... */