fix bug when using get_deleted_objects function

[mymusise]

Fix issue #622 in django2.

[Flamingo-Chen]

    if django_version > (2, 1):
        deletable_objects, model_count, perms_needed, protected = get_deleted_objects(
            queryset, self.opts, self.admin_site)
    else:
        using = router.db_for_write(self.model)
        deletable_objects, model_count, perms_needed, protected = get_deleted_objects(
            queryset, self.opts, self.user, self.admin_site, using)