posix-timers: fix deletion race

timer_delete does:
	lock_timer();
	timer->it_process = NULL;
	unlock_timer();
	release_posix_timer();

timer->it_process is checked in lock_timer() to prevent access to a
timer, which is on the way to be deleted, but the check happens after
idr_lock is dropped. This allows release_posix_timer() to delete the
timer before the lock code can check the timer:

  CPU 0				CPU 1

  lock_timer();
  timer->it_process = NULL;
  unlock_timer();
				lock_timer()
					spin_lock(idr_lock);
					timer = idr_find();
					spin_lock(timer->lock);
					spin_unlock(idr_lock);
  release_posix_timer();
	spin_lock(idr_lock);
	idr_remove(timer);
	spin_unlock(idr_lock);
	free_timer(timer);
					if (timer->......)

Change the locking to prevent this.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>