oom: don't count on mm-less current process

out_of_memory() doesn't trigger the OOM killer if the current task is
already exiting or it has fatal signals pending, and gives the task
access to memory reserves instead.  However, doing so is wrong if
out_of_memory() is called by an allocation (e.g. from exit_task_work())
after the current task has already released its memory and cleared
TIF_MEMDIE at exit_mm().  If we again set TIF_MEMDIE to post-exit_mm()
current task, the OOM killer will be blocked by the task sitting in the
final schedule() waiting for its parent to reap it.  It will trigger an
OOM livelock if its parent is unable to reap it due to doing an
allocation and waiting for the OOM killer to kill it.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: David Rientjes <rientjes@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>