hammer2 - Work on concurrent bulkfree stability

hammer2 - Work on concurrent bulkfree stability

* The dedup bits in the DIO structure must be set atomically with
  the setting of the bitmap bits in the freemap to avoid racing
  bulkfree.

* dedup bits are normally deleted on the 11->10 transition in bulkfree,
  and asserted to be deleted on the 10->00 transition. Also delete
  dedup bits when a modified chain with no parent is destroyed, and
  for a chain's prior data reference when it is being replaced by a
  new data reference.

* Clean up the DIO dedup management code.

* Clean up the DIO allocation code.

* Cap the size of the DIO cache to 100000 elements for now, if nbuf * 2
  is greater.