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.