pk: Use saner magic flag for allocated-but-not-written hoard chunks
pk: Use saner magic flag for allocated-but-not-written hoard chunks

- Rather than using (non-null length + null tag) as the flag for chunk
slots which have been allocated by a PK process but not yet written, use
(null tag + referenced flag set), which makes more sense

- When initially allocating a chunk slot, set only the referenced flag,
and set all other metadata after the chunk write has finished

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