[BLOCK] kill generic max_back_kb handling

This patch kills max_back_kb handling from elv_dispatch_sort() and
kills max_back_kb field from struct request_queue.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <axboe@suse.de>

drivers/block/elevator.c

@@ -136,7 +136,6 @@ static int elevator_attach(request_queue_t *q, struct elevator_type *e,
 	q->elevator = eq;
 	q->end_sector = 0;
 	q->boundary_rq = NULL;
-	q->max_back_kb = 0;
 
 	if (eq->ops->elevator_init_fn)
 		ret = eq->ops->elevator_init_fn(q, eq);
@@ -227,15 +226,12 @@ void elevator_exit(elevator_t *e)
 void elv_dispatch_sort(request_queue_t *q, struct request *rq)
 {
 	sector_t boundary;
-	unsigned max_back;
 	struct list_head *entry;
 
 	if (q->last_merge == rq)
 		q->last_merge = NULL;
 
 	boundary = q->end_sector;
-	max_back = q->max_back_kb * 2;
-	boundary = boundary > max_back ? boundary - max_back : 0;
 
 	list_for_each_prev(entry, &q->queue_head) {
 		struct request *pos = list_entry_rq(entry);

include/linux/blkdev.h

@@ -339,7 +339,6 @@ struct request_queue
 	 */
 	sector_t		end_sector;
 	struct request		*boundary_rq;
-	unsigned int		max_back_kb;
 
 	/*
 	 * Auto-unplugging state