The attachment modifies the SG_IO ioctl that is in
the block layer to:
   - convey the SCSI status value back via the sg_io_hdr
     structure
   - in the event of CHECK CONDITION, convey the sense
     buffer back via the sg_io_hdr structure
   - set "output" fields in sg_io_hdr structure to sane
     values
   - modify the errno behaviour to be more like the
     SCSI generic driver's SG_IO ioctl

This patch was presented around lk 2.5.51 but fell between
the cracks. The only modification is to take account of
the changes to scsi/scsi_lib.c since then.