hammer2 - stabilization

hammer2 - stabilization

* Fix the hammer2_chain_assert_not_present() call in hammer2_chain_insert()
  to avoid SMP races that could cause it to trigger falsely.  Also fix a
  case where inserting a deleted chain was improperly calling it.

* Fix bugs in hammer2_combined_find().  It is possible for a deleted
  chain from a prior synchronization point to cover a matching media
  blockref from a later synchronization point.  When both a media block
  and an in-memory chain are found, do not just blindly choose the
  in-memory chain.  Choose based on the synchronization points.

* Refactor part of the hardlink code.  Do not create a dummy chain
  to place-hold the target when a hardlink has to be shifted up to
  a parent directory.  Allow hammer2_chain_duplicate() to do it.