Fix `memsize_of_all` to count memory of subclass instances; #5040

`ObjectSpace.memsize_of_all` takes a class and count memory size of all
instances of the class and its subclasses (if any).

mrbgems/mruby-objectspace/src/mruby_objectspace.c

@@ -353,7 +353,8 @@ static int
 os_memsize_of_all_cb(mrb_state *mrb, struct RBasic *obj, void *d)
 {
   struct os_memsize_of_all_cb_data *data = (struct os_memsize_of_all_cb_data *)d;
-  if(mrb_class_real(obj->c) == data->type) data->t += os_memsize_of_object(mrb, mrb_obj_value(obj));
+  if(mrb_obj_is_kind_of(mrb, mrb_obj_value(obj), data->type))
+    data->t += os_memsize_of_object(mrb, mrb_obj_value(obj));
   return 0;
 }