scsi: sd: sr: Convert two assignments into warning statements

Before scsi_prep_fn() calls the ULP .init_command() callback
function it stores the SCSI command pointer in request.special.
This means that the SCpnt = rq->special assignments in the sd
and sr drivers assign a pointer to itself. Hence convert these
two assignment statements into warning statements.

Signed-off-by: Bart Van Assche <[e-mail]>
Reviewed-by: Hannes Reinecke <[e-mail]>
Reviewed-by: Johannes Thumshirn <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/sd.c

···
 	ret = scsi_init_io(SCpnt);
 	if (ret != BLKPREP_OK)
 		goto out;
-	SCpnt = rq->special;
+	WARN_ON_ONCE(SCpnt != rq->special);
 
 	/* from here on until we're complete, any goto out
 	 * is used for a killable error condition */

drivers/scsi/sr.c

···
 	ret = scsi_init_io(SCpnt);
 	if (ret != BLKPREP_OK)
 		goto out;
-	SCpnt = rq->special;
+	WARN_ON_ONCE(SCpnt != rq->special);
 	cd = scsi_cd(rq->rq_disk);
 
 	/* from here on until we're complete, any goto out