usb-storage: report underflow with no sense data

This patch (as1118) addresses a problem with certain USB mass-storage
devices.  These devices sometimes return less data than asked for and
then provide no sense data to explain the problem.  Currently
usb-storage leaves it up to the SCSI layer to decide how this should
be handled, and the SCSI layer interprets the lack of sense data to
mean that nothing went wrong.  But if we got less data than required
then something definitely _did_ go wrong, and we should say so.

The patch tells the SCSI layer to retry the command when this sort of
thing happens.  Retrying may not solve the underlying problem, but
it's better than believing that data was transferred when it wasn't.
Signed-off-by: NAlan Stern <stern@rowland.harvard.edu>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/storage/transport.c

@@ -663,7 +663,7 @@ void usb_stor_invoke_transport(struct scsi_cmnd *srb, struct us_data *us)
 	}
 
 	/* Did we transfer less than the minimum amount required? */
-	if (srb->result == SAM_STAT_GOOD &&
+	if ((srb->result == SAM_STAT_GOOD || srb->sense_buffer[2] == 0) &&
 			scsi_bufflen(srb) - scsi_get_resid(srb) < srb->underflow)
 		srb->result = (DID_ERROR << 16) | (SUGGEST_RETRY << 24);