[SCSI] sg: use idr to replace static arrays

sg uses a scheme to reallocate a single contiguous array of all its
pointers for lookup and management.  This didn't matter too much when sg
could only attach 256 nodes, but now the maximum has been bumped up to
32k we're starting to push the limits of the maximum allocatable
contiguous memory.  The solution to this is to eliminate the static
array and do everything via idr, which this patch does.
Acked-by: Douglas Gilbert <dougg@torque.net>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>