pk: Update hoard_get_chunk() assumptions for hoard cache compaction
pk: Update hoard_get_chunk() assumptions for hoard cache compaction

gchoard mode will grow an option to compact the hoard cache, moving
chunks at high offsets into empty slots at lower offsets.  Update some
assumptions in the existing hoard code:

- No longer print a warning from _hoard_invalidate_chunk() if the chunk
we're invalidating is in fact not present.  If we call hoard_get_chunk()
while compaction is in progress, this case is not unlikely, so don't
make noise about it.

- If hoard_get_chunk() finds an invalid chunk, have it check again
after invalidating the chunk rather than returning an error outright.
The chunk may have been moved by hoard cache compaction, so the second
attempt may well succeed.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr