[S390] reference bit testing for unmapped pages

On x86 a page without a mapper is by definition not referenced / old.
The s390 architecture keeps the reference bit in the storage key and
the current code will check the storage key for page without a mapper.
This leads to an interesting effect: the first time an s390 system
needs to write pages to swap it only finds referenced pages. This
causes a lot of pages to get added and written to the swap device.
To avoid this behaviour change page_referenced to query the storage
key only if there is a mapper of the page.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

mm/rmap.c

@@ -869,11 +869,11 @@ int page_referenced(struct page *page,
 								vm_flags);
 		if (we_locked)
 			unlock_page(page);
+
+		if (page_test_and_clear_young(page_to_pfn(page)))
+			referenced++;
 	}
 out:
-	if (page_test_and_clear_young(page_to_pfn(page)))
-		referenced++;
-
 	return referenced;
 }