SCSI: Fix error handling when no ULD is attached

Commit 18a4d0a2 ("[SCSI] Handle disk devices which can not process
medium access commands") introduced a bug in which we would attempt to
dereference the scsi driver even when the device had no ULD attached.

Ensure that a driver is registered and make the driver accessor function
more resilient to errors during device discovery.
Reported-by: NElric Fu <elricfu1@gmail.com>
Reported-by: NBart Van Assche <bvanassche@acm.org>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/scsi/scsi_error.c

@@ -835,7 +835,7 @@ static int scsi_send_eh_cmnd(struct scsi_cmnd *scmd, unsigned char *cmnd,
 
 	scsi_eh_restore_cmnd(scmd, &ses);
 
-	if (sdrv->eh_action)
+	if (sdrv && sdrv->eh_action)
 		rtn = sdrv->eh_action(scmd, cmnd, cmnd_size, rtn);
 
 	return rtn;

include/scsi/scsi_cmnd.h

@@ -134,6 +134,9 @@ struct scsi_cmnd {
 
 static inline struct scsi_driver *scsi_cmd_to_driver(struct scsi_cmnd *cmd)
 {
+	if (!cmd->request->rq_disk)
+		return NULL;
+
 	return *(struct scsi_driver **)cmd->request->rq_disk->private_data;
 }