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/block/mg_disk.c b/drivers/block/mg_disk.c
index b82c5ce..76fa3de 100644
--- a/drivers/block/mg_disk.c
+++ b/drivers/block/mg_disk.c
@@ -974,8 +974,7 @@
 	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__);