mm: fix incorrect variable type in do_try_to_free_pages()

"Smarter retry of costly-order allocations" patch series change behaver of
do_try_to_free_pages().  But unfortunately ret variable type was
unchanged.

Thus an overflow is possible.
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -1307,7 +1307,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
 					struct scan_control *sc)
 {
 	int priority;
-	int ret = 0;
+	unsigned long ret = 0;
 	unsigned long total_scanned = 0;
 	unsigned long nr_reclaimed = 0;
 	struct reclaim_state *reclaim_state = current->reclaim_state;