elevator_add_req_fn* called to add a new request into the scheduler
-elevator_queue_empty_fn returns true if the merge queue is empty.
- Drivers shouldn't use this, but rather check
- if elv_next_request is NULL (without losing the
- request if one exists!)
-
elevator_former_req_fn
elevator_latter_req_fn These return the request before or after the
one specified in disk sort order. Used by the