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>

drivers/scsi/libfc/fc_lport.c

@@ -2095,7 +2095,7 @@ int fc_lport_bsg_request(struct bsg_job *job)
 
 	bsg_reply->reply_payload_rcv_len = 0;
 	if (rsp)
-		rsp->resid_len = job->reply_payload.payload_len;
+		scsi_req(rsp)->resid_len = job->reply_payload.payload_len;
 
 	mutex_lock(&lport->lp_mutex);