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>
2 files changed