[S390] incorrect PageTables counter for kvm page tables

The page_table_free_pgste function is used for kvm processes to free page
tables that have the pgste extension. It calls pgtable_page_ctor instead of
pgtable_page_dtor which increases NR_PAGETABLE instead of decreasing it.
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/pgtable.c

@@ -574,7 +574,7 @@ static inline void page_table_free_pgste(unsigned long *table)
 	page = pfn_to_page(__pa(table) >> PAGE_SHIFT);
 	mp = (struct gmap_pgtable *) page->index;
 	BUG_ON(!list_empty(&mp->mapper));
-	pgtable_page_ctor(page);
+	pgtable_page_dtor(page);
 	atomic_set(&page->_mapcount, -1);
 	kfree(mp);
 	__free_page(page);