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
1abec4fd.
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

drivers/block/mg_disk.c

@@ -974,8 +974,7 @@ static int mg_probe(struct platform_device *plat_dev)
 	host->breq->queuedata = host;
 
 	/* mflash is random device, thanx for the noop */
-	elevator_exit(host->breq->elevator);
-	err = elevator_init(host->breq, "noop");
+	err = elevator_change(host->breq, "noop");
 	if (err) {
 		printk(KERN_ERR "%s:%d (elevator_init) fail\n",
 				__func__, __LINE__);

drivers/s390/char/tape_block.c

@@ -217,8 +217,7 @@ tapeblock_setup_device(struct tape_device * device)
 	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;