scsi-mq: fix requests that use a separate CDB buffer

This patch fixes code such as the following with scsi-mq enabled:

    rq = blk_get_request(...);
    blk_rq_set_block_pc(rq);

    rq->cmd = my_cmd_buffer; /* separate CDB buffer */

    blk_execute_rq_nowait(...);

Code like this appears in e.g. sg_start_req() in drivers/scsi/sg.c (for
large CDBs only).  Without this patch, scsi_mq_prep_fn() will set
rq->cmd back to rq->__cmd, causing the wrong CDB to be sent to the device.
Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-core.c

@@ -1252,7 +1252,6 @@ void blk_rq_set_block_pc(struct request *rq)
 	rq->__sector = (sector_t) -1;
 	rq->bio = rq->biotail = NULL;
 	memset(rq->__cmd, 0, sizeof(rq->__cmd));
-	rq->cmd = rq->__cmd;
 }
 EXPORT_SYMBOL(blk_rq_set_block_pc);
 

block/blk-mq.c

@@ -176,6 +176,8 @@ static void blk_mq_rq_ctx_init(struct request_queue *q, struct blk_mq_ctx *ctx,
 	/* tag was already set */
 	rq->errors = 0;
 
+	rq->cmd = rq->__cmd;
+
 	rq->extra_len = 0;
 	rq->sense_len = 0;
 	rq->resid_len = 0;

drivers/scsi/scsi_lib.c

@@ -1808,7 +1808,6 @@ static int scsi_mq_prep_fn(struct request *req)
 
 	cmd->tag = req->tag;
 
-	req->cmd = req->__cmd;
 	cmd->cmnd = req->cmd;
 	cmd->prot_op = SCSI_PROT_NORMAL;