vmscan: fix do_try_to_free_pages() return value when priority==0 reclaim failure

Greg Thelen reported recent Johannes's stack diet patch makes kernel hang.
 His test is following.

  mount -t cgroup none /cgroups -o memory
  mkdir /cgroups/cg1
  echo $$ > /cgroups/cg1/tasks
  dd bs=1024 count=1024 if=/dev/null of=/data/foo
  echo $$ > /cgroups/tasks
  echo 1 > /cgroups/cg1/memory.force_empty

Actually, This OOM hard to try logic have been corrupted since following
two years old patch.

	commit a41f24ea
	Author: Nishanth Aravamudan <nacc@us.ibm.com>
	Date:   Tue Apr 29 00:58:25 2008 -0700

	    page allocator: smarter retry of costly-order allocations

Original intention was "return success if the system have shrinkable zones
though priority==0 reclaim was failure".  But the above patch changed to
"return nr_reclaimed if .....".  Oh, That forgot nr_reclaimed may be 0 if
priority==0 reclaim failure.

And Johannes's patch 0aeb2339 ("vmscan: remove all_unreclaimable scan
control") made it more corrupt.  Originally, priority==0 reclaim failure
on memcg return 0, but this patch changed to return 1.  It totally
confused memcg.

This patch fixes it completely.

Reported-by: Greg Thelen <gthelen@google.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Tested-by: Greg Thelen <gthelen@google.com>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>