Query join to retrieve index links is incorrect for xmlsitemap_menu

As part of xmlsitemap_menu_update_6201, the type stored for xmlsitemap menu items was renamed from `menu` to `menu_link`.

The query in `xmlsitemap_menu_xmlsitemap_index_links` hasn't been updated accordingly.

One can verify which type name should be used on any xmlsitemap installation, by looking at the xmlsitemap table and confirming that all entries have a type of `menu_link` and there are none for `menu`.

As a result, when cron calls to xmlsitemap_menu_xmlsitemap_index_links are made, it will always return the first $batch_limit number of entries, even though these have already been processed.