[AIO] fix cancellation of aio_down()

aio_down() can only be cancelled once.  Clear the ki_cancel method when 
the cancellation is complete.

Signed-off-by: Benjamin LaHaise <bcrl@linux.intel.com>
diff -purN --exclude=description 50_aio_down_cancel/lib/semaphore-sleepers.c 51_aio_down_cancel_fix/lib/semaphore-sleepers.c
--- 50_aio_down_cancel/lib/semaphore-sleepers.c	2005-08-08 17:15:39.000000000 -0400
+++ 51_aio_down_cancel_fix/lib/semaphore-sleepers.c	2005-08-12 12:44:31.000000000 -0400
@@ -130,6 +130,7 @@ static int cancel_aio_down(struct kiocb 
 	if (!list_empty(&iocb->ki_wait.task_list)) {
 		/* Ensure aio_down_wait() can no longer be called. */
 		list_del_init(&iocb->ki_wait.task_list);
+		iocb->ki_cancel = NULL;
 		fixup_down_trylock_locked(sem);
 		event->res = is_sync_kiocb(iocb) ? -ERESTARTSYS : -EINTR;
 	} else