oom, oom_reaper: protect oom_reaper_list using simpler way

"oom, oom_reaper: disable oom_reaper for oom_kill_allocating_task" tried
to protect oom_reaper_list using MMF_OOM_KILLED flag.  But we can do it
by simply checking tsk->oom_reaper_list != NULL.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 8cc55f0..b34d279 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -547,7 +547,7 @@
 
 static void wake_oom_reaper(struct task_struct *tsk)
 {
-	if (!oom_reaper_th)
+	if (!oom_reaper_th || tsk->oom_reaper_list)
 		return;
 
 	get_task_struct(tsk);
@@ -681,7 +681,7 @@
 	unsigned int victim_points = 0;
 	static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
 					      DEFAULT_RATELIMIT_BURST);
-	bool can_oom_reap;
+	bool can_oom_reap = true;
 
 	/*
 	 * If the task is already exiting, don't alarm the sysadmin or kill
@@ -743,10 +743,6 @@
 	/* Get a reference to safely compare mm after task_unlock(victim) */
 	mm = victim->mm;
 	atomic_inc(&mm->mm_count);
-
-	/* Make sure we do not try to oom reap the mm multiple times */
-	can_oom_reap = !test_and_set_bit(MMF_OOM_KILLED, &mm->flags);
-
 	/*
 	 * We should send SIGKILL before setting TIF_MEMDIE in order to prevent
 	 * the OOM victim from depleting the memory reserves from the user