block: switch s390 tape_block and mg_disk to elevator_change()

Now that we have this API, switch the two in-kernel users to it.
Resolves an oops introduced by commit
1abec4fdbb142e3ccb6ce99832fae42129134a96.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/drivers/s390/char/tape_block.c b/drivers/s390/char/tape_block.c
index b7de025..85cf607 100644
--- a/drivers/s390/char/tape_block.c
+++ b/drivers/s390/char/tape_block.c
@@ -217,8 +217,7 @@
 	if (!blkdat->request_queue)
 		return -ENOMEM;
 
-	elevator_exit(blkdat->request_queue->elevator);
-	rc = elevator_init(blkdat->request_queue, "noop");
+	rc = elevator_change(blkdat->request_queue, "noop");
 	if (rc)
 		goto cleanup_queue;