A little more testing of st's SG_IO ioctl turned up a
small problem.

This is the corresponding patch that was applied to the
sd driver when it received the block layer SG_IO ioctl.

For least surprise of lk 2.4 utilities that use the
SCSI_IOCTL_GET_IDLUN and SCSI_IOCTL_GET_BUS_NUMBER
ioctls (e.g. sg_map) it is better to return the correct
values rather than 0.