wait_task_inactive(): don't consider task->nivcsw

If wait_task_inactive() returns success the task was deactivated.  In that
case schedule() always increments ->nvcsw which alone can be used as a
"generation counter".

If the next call returns the same number, we can be sure that the task was
unscheduled.  Otherwise, because we know that .on_rq == 0 again, ->nvcsw
should have been changed in between.

Q: perhaps it is better to do "ncsw = (p->nvcsw << 1) | 1" ?  This
decreases the possibility of "was it unscheduled" false positive when
->nvcsw == 0.

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>