The shuffle descriptors need to be unused for input *and* output

The shuffle descriptors need to be unused for input *and* output

The memory used to hold shuffle descriptors must be unused during the
entire execution of the shuffle sequence, and therefore needs to be
free at input *and* output time.

Once we have picked a region for the shuffle descriptors, we can free
this working copy and start over with a clean memory map, the only
thing we do there is mark as reserved the memory used by the
descriptors.