freezer: fix PF_NOFREEZE vs freezeable race

This patch fixes the race pointed out by Oleg Nesterov.

* Freezer marks a thread as freezeable.
* The thread now marks itself PF_NOFREEZE, but it will be frozen on
  on calling try_to_freeze(). Thus the task is frozen, even though it doesn't
  want to.
* Subsequent thaw_processes() will also fail to thaw the task since it is
  marked PF_NOFREEZE.

Avoid this problem by checking the task's PF_NOFREEZE status in
frozen_processes() before marking the task as frozen.

Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>