hammer2 - Fix misc bugs

hammer2 - Fix misc bugs

* Move the live_zero optimization from hammer2_chain to
  hammer2_chain_core.  It is only applicable to the core
  and delete-duplicate operations can mess up the cache.

* Move the HAMMER2_CHAIN_COUNTEDBREFS flag to HAMMER2_CORE_COUNTEDBREFS.
  It is only applicable to the core and delete-duplication operations
  can really mess up calculations of live_count otherwise.

* Don't bump live_count if inserting a deleted chain.

* The vp in the hammer2_sync_scan2() is not locked on purpose.  Use the
  synclist token interlock to safely ref the hammer2_inode before
  potentially blocking, otherwise it can get ripped out from under us.