scsi: sr: 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/[e-mail]
Cc: Christoph Hellwig <[e-mail]>
Cc: Hannes Reinecke <[e-mail]>
Cc: Ming Lei <[e-mail]>
Reviewed-by: Hannes Reinecke <[e-mail]>
Signed-off-by: Bart Van Assche <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/sr.c

···
 	int good_bytes = (result == 0 ? this_count : 0);
 	int block_sectors = 0;
 	long error_sector;
-	struct scsi_cd *cd = scsi_cd(SCpnt->request->rq_disk);
+	struct request *rq = scsi_cmd_to_rq(SCpnt);
+	struct scsi_cd *cd = scsi_cd(rq->rq_disk);
 
 #ifdef DEBUG
 	scmd_printk(KERN_INFO, SCpnt, "done: %x\n", result);
···
 				break;
 			error_sector =
 				get_unaligned_be32(&SCpnt->sense_buffer[3]);
-			if (SCpnt->request->bio != NULL)
-				block_sectors =
-					bio_sectors(SCpnt->request->bio);
+			if (rq->bio != NULL)
+				block_sectors = bio_sectors(rq->bio);
 			if (block_sectors < 4)
 				block_sectors = 4;
 			if (cd->device->sector_size == 2048)
 				error_sector <<= 2;
 			error_sector &= ~(block_sectors - 1);
-			good_bytes = (error_sector -
-				      blk_rq_pos(SCpnt->request)) << 9;
+			good_bytes = (error_sector - blk_rq_pos(rq)) << 9;
 			if (good_bytes < 0 || good_bytes >= this_count)
 				good_bytes = 0;
 			/*
···
 {
 	int block = 0, this_count, s_size;
 	struct scsi_cd *cd;
-	struct request *rq = SCpnt->request;
+	struct request *rq = scsi_cmd_to_rq(SCpnt);
 	blk_status_t ret;
 
 	ret = scsi_alloc_sgtables(SCpnt);