mm/vmscan.c: fix mapping use after free

We need lock_page_nosync() here because we have no reference to the
mapping when taking the page lock.
Signed-off-by: NNick Piggin <npiggin@suse.de>
Reviewed-by: NJohannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -298,7 +298,7 @@ static int may_write_to_queue(struct backing_dev_info *bdi)
 static void handle_write_error(struct address_space *mapping,
 				struct page *page, int error)
 {
-	lock_page(page);
+	lock_page_nosync(page);
 	if (page_mapping(page) == mapping)
 		mapping_set_error(mapping, error);
 	unlock_page(page);