[SCSI] make use of the residue value

USB sometimes doesn't return an error but instead returns a residue
value indicating part (or all) of the command wasn't completed.  So if
the driver _done() error processing indicates the command was fully
processed, subtract off the residue so that this USB error gets
propagated.

Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

drivers/scsi/scsi.c

@@ -855,9 +855,18 @@ void scsi_finish_command(struct scsi_cmnd *cmd)
 
 	good_bytes = scsi_bufflen(cmd);
         if (cmd->request->cmd_type != REQ_TYPE_BLOCK_PC) {
+		int old_good_bytes = good_bytes;
 		drv = scsi_cmd_to_driver(cmd);
 		if (drv->done)
 			good_bytes = drv->done(cmd);
+		/*
+		 * USB may not give sense identifying bad sector and
+		 * simply return a residue instead, so subtract off the
+		 * residue if drv->done() error processing indicates no
+		 * change to the completion length.
+		 */
+		if (good_bytes == old_good_bytes)
+			good_bytes -= scsi_get_resid(cmd);
 	}
 	scsi_io_completion(cmd, good_bytes);
 }