sched: Fix schedule_tail() to disable preemption

finish_task_switch() enables preemption, so post_schedule(rq) can be
called on the wrong (and even dead) CPU. Afaics, nothing really bad
can happen, but in this case we can wrongly clear rq->post_schedule
on that CPU. And this simply looks wrong in any case.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Kirill Tkhai <tkhai@yandex.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20141008193644.GA32055@redhat.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>