scsi: scsi_transport_spi: Use scsi_cmd_to_rq() instead of scsi_cmnd.request

Prepare for removal of the request pointer by using scsi_cmd_to_rq()
instead. This patch does not change any functionality.

Link: https://lore.kernel.org/r/20210809230355.8186-7-bvanassche@acm.org
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_transport_spi.c

@@ -1230,7 +1230,7 @@ int spi_populate_tag_msg(unsigned char *msg, struct scsi_cmnd *cmd)
 {
         if (cmd->flags & SCMD_TAGGED) {
 		*msg++ = SIMPLE_QUEUE_TAG;
-        	*msg++ = cmd->request->tag;
+		*msg++ = scsi_cmd_to_rq(cmd)->tag;
         	return 2;
 	}