wait_task_inactive: "improve" the returned value for ->nvcsw == 0

wait_task_inactive() returns 1 when p->nvcsw == 0 || p->nvcsw == 1.  This
means that two subsequent calls can return the same number while the task
was scheduled in between.

Change the code to return "nvcsw | LONG_MIN" instead of "nvcsw ?: 1", now
the overlap always needs LONG_MAX schedules.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched.c

@@ -1922,7 +1922,7 @@ unsigned long wait_task_inactive(struct task_struct *p, long match_state)
 		on_rq = p->se.on_rq;
 		ncsw = 0;
 		if (!match_state || p->state == match_state)
-			ncsw = p->nvcsw ?: 1;
+			ncsw = p->nvcsw | LONG_MIN; /* sets MSB */
 		task_rq_unlock(rq, &flags);
 
 		/*