mm: postpone migrated page mapping reset

Postpone resetting page->mapping until the final remove_migration_ptes().
Otherwise the expression PageAnon(migration_entry_to_page(entry)) does not
work.
Signed-off-by: NKonstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -445,7 +445,6 @@ void migrate_page_copy(struct page *newpage, struct page *page)
 	ClearPageSwapCache(page);
 	ClearPagePrivate(page);
 	set_page_private(page, 0);
-	page->mapping = NULL;
 
 	/*
 	 * If any waiters have accumulated on the new page then
@@ -667,6 +666,7 @@ static int move_to_new_page(struct page *newpage, struct page *page,
 	} else {
 		if (remap_swapcache)
 			remove_migration_ptes(page, newpage);
+		page->mapping = NULL;
 	}
 
 	unlock_page(newpage);