cgroups: fix a css_set not found bug in cgroup_attach_proc

There is a BUG when migrating a PF_EXITING proc. Since css_set_prefetch()
is not called for the PF_EXITING case, find_existing_css_set() will return
NULL inside cgroup_task_migrate() causing a BUG.

This bug is easy to reproduce. Create a zombie and echo its pid to
cgroup.procs.

$ cat zombie.c
\#include <unistd.h>

int main()
{
  if (fork())
      pause();
  return 0;
}
$

We are hitting this bug pretty regularly on ChromeOS.

This bug is already fixed by Tejun Heo's cgroup patchset which is
targetted for the next merge window:

https://lkml.org/lkml/2011/11/1/356



I've create a smaller patch here which just fixes this bug so that a
fix can be merged into the current release and stable.

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Downstream-Bug-Report: http://crosbug.com/23953


Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: containers@lists.linux-foundation.org
Cc: cgroups@vger.kernel.org
Cc: stable@kernel.org
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Menage <paul@paulmenage.org>
Cc: Olof Johansson <olofj@chromium.org>