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>