block: fold cmd_type into the REQ_OP_ space

Instead of keeping two levels of indirection for requests types, fold it
all into the operations.  The little caveat here is that previously
cmd_type only applied to struct request, while the request and bio op
fields were set to plain REQ_OP_READ/WRITE even for passthrough
operations.

Instead this patch adds new REQ_OP_* for SCSI passthrough and driver
private requests, althought it has to add two for each so that we
can communicate the data in/out nature of the request.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
index 226a8de..e0e308b 100644
--- a/drivers/scsi/sg.c
+++ b/drivers/scsi/sg.c
@@ -1698,7 +1698,8 @@ sg_start_req(Sg_request *srp, unsigned char *cmd)
 	 * With scsi-mq disabled, blk_get_request() with GFP_KERNEL usually
 	 * does not sleep except under memory pressure.
 	 */
-	rq = blk_get_request(q, rw, GFP_KERNEL);
+	rq = blk_get_request(q, hp->dxfer_direction == SG_DXFER_TO_DEV ?
+			REQ_OP_SCSI_OUT : REQ_OP_SCSI_IN, GFP_KERNEL);
 	if (IS_ERR(rq)) {
 		kfree(long_cmdp);
 		return PTR_ERR(rq);