block: kill rq->data

Now that all block request data transfer is done via bio, rq->data
isn't used.  Kill it.

While at it, make the roles of rq->special and buffer clear.

[ Impact: drop now unncessary field from struct request ]

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Boaz Harrosh <bharrosh@panasas.com>
diff --git a/block/blk-core.c b/block/blk-core.c
index 0520cc7..6dd180c 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -189,10 +189,9 @@
 						(unsigned long long)rq->sector,
 						rq->nr_sectors,
 						rq->current_nr_sectors);
-	printk(KERN_INFO "  bio %p, biotail %p, buffer %p, data %p, len %u\n",
+	printk(KERN_INFO "  bio %p, biotail %p, buffer %p, len %u\n",
 						rq->bio, rq->biotail,
-						rq->buffer, rq->data,
-						rq->data_len);
+						rq->buffer, rq->data_len);
 
 	if (blk_pc_request(rq)) {
 		printk(KERN_INFO "  cdb: ");