scsi: sg: fix SG_DXFER_FROM_DEV transfers

SG_DXFER_FROM_DEV transfers do not necessarily have a dxferp as we set
it to NULL for the old sg_io read/write interface, but must have a
length bigger than 0. This fixes a regression introduced by commit
28676d869bbb ("scsi: sg: check for valid direction before starting the
request")

Signed-off-by: Johannes Thumshirn <[e-mail]>
Fixes: 28676d869bbb ("scsi: sg: check for valid direction before starting the request")
Reported-by: Chris Clayton <[e-mail]>
Tested-by: Chris Clayton <[e-mail]>
Cc: Douglas Gilbert <[e-mail]>
Reviewed-by: Hannes Reinecke <[e-mail]>
Tested-by: Chris Clayton <[e-mail]>
Acked-by: Douglas Gilbert <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/sg.c

···
 		if (hp->dxferp || hp->dxfer_len > 0)
 			return false;
 		return true;
-	case SG_DXFER_TO_DEV:
 	case SG_DXFER_FROM_DEV:
+		if (hp->dxfer_len < 0)
+			return false;
+		return true;
+	case SG_DXFER_TO_DEV:
 	case SG_DXFER_TO_FROM_DEV:
 		if (!hp->dxferp || hp->dxfer_len == 0)
 			return false;