s390: use pfn_swap_entry_folio() in ptep_zap_swap_entry()

Call pfn_swap_entry_folio() in ptep_zap_swap_entry() as preparation for
converting mm counter functions to take a folio.

Link: https://lkml.kernel.org/r/20240111152429.3374566-5-willy@infradead.orgSigned-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/s390/mm/pgtable.c

@@ -721,9 +721,9 @@ static void ptep_zap_swap_entry(struct mm_struct *mm, swp_entry_t entry)
 	if (!non_swap_entry(entry))
 		dec_mm_counter(mm, MM_SWAPENTS);
 	else if (is_migration_entry(entry)) {
-		struct page *page = pfn_swap_entry_to_page(entry);
+		struct folio *folio = pfn_swap_entry_folio(entry);
 
-		dec_mm_counter(mm, mm_counter(page));
+		dec_mm_counter(mm, mm_counter(&folio->page));
 	}
 	free_swap_and_cache(entry);
 }