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/include/linux/blkdev.h b/include/linux/blkdev.h
index cfeb3c2..12c545e 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -211,8 +211,8 @@
 
 	unsigned short ioprio;
 
-	void *special;
-	char *buffer;
+	void *special;		/* opaque pointer available for LLD use */
+	char *buffer;		/* kaddr of the current segment if available */
 
 	int tag;
 	int errors;
@@ -229,7 +229,6 @@
 	unsigned int data_len;
 	unsigned int extra_len;	/* length of alignment and padding */
 	unsigned int sense_len;
-	void *data;
 	void *sense;
 
 	unsigned long deadline;