block: split scsi_request out of struct request

And require all drivers that want to support BLOCK_PC to allocate it
as the first thing of their private data.  To support this the legacy
IDE and BSG code is switched to set cmd_size on their queues to let
the block layer allocate the additional space.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/ide/ide-devsets.c b/drivers/ide/ide-devsets.c
index 0dd43b4..fd56c9dd 100644
--- a/drivers/ide/ide-devsets.c
+++ b/drivers/ide/ide-devsets.c
@@ -166,10 +166,11 @@ int ide_devset_execute(ide_drive_t *drive, const struct ide_devset *setting,
 		return setting->set(drive, arg);
 
 	rq = blk_get_request(q, READ, __GFP_RECLAIM);
+	scsi_req_init(rq);
 	rq->cmd_type = REQ_TYPE_DRV_PRIV;
-	rq->cmd_len = 5;
-	rq->cmd[0] = REQ_DEVSET_EXEC;
-	*(int *)&rq->cmd[1] = arg;
+	scsi_req(rq)->cmd_len = 5;
+	scsi_req(rq)->cmd[0] = REQ_DEVSET_EXEC;
+	*(int *)&scsi_req(rq)->cmd[1] = arg;
 	rq->special = setting->set;
 
 	if (blk_execute_rq(q, NULL, rq, 0))
@@ -183,7 +184,7 @@ ide_startstop_t ide_do_devset(ide_drive_t *drive, struct request *rq)
 {
 	int err, (*setfunc)(ide_drive_t *, int) = rq->special;
 
-	err = setfunc(drive, *(int *)&rq->cmd[1]);
+	err = setfunc(drive, *(int *)&scsi_req(rq)->cmd[1]);
 	if (err)
 		rq->errors = err;
 	ide_complete_rq(drive, err, blk_rq_bytes(rq));