set EXIT_DEAD state in do_exit(), not in schedule()

schedule() checks PF_DEAD on every context switch and sets ->state = EXIT_DEAD to ensure that the exiting task will be deactivated. Note that this EXIT_DEAD is in fact a "random" value, we can use any bit except normal TASK_XXX values.

It is better to set this state in do_exit() along with PF_DEAD flag and remove that check in schedule().

We are safe wrt concurrent try_to_wake_up() (for example ptrace, tkill), it can not change task's ->state: the 'state' argument of try_to_wake_up() can't have EXIT_DEAD bit. And in case when try_to_wake_up() sees a stale value of ->state == TASK_RUNNING it will do nothing.

@@ -957,6 +957,7 @@ fastcall NORET_TYPE void do_exit(long co
 	preempt_disable();
 	BUG_ON(tsk->flags & PF_DEAD);
 	tsk->flags |= PF_DEAD;
+	tsk->state = EXIT_DEAD;
 
 	schedule();
 	BUG();

@@ -3311,9 +3311,6 @@ need_resched_nonpreemptible:
 
 	spin_lock_irq(&rq->lock);
 
-	if (unlikely(prev->flags & PF_DEAD))
-		prev->state = EXIT_DEAD;
-
 	switch_count = &prev->nivcsw;
 	if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
 		switch_count = &prev->nvcsw;