hammer2 - Add assertions, fix a flush race against delete-duplicate

hammer2 - Add assertions, fix a flush race against delete-duplicate

* hammer2_chain_delete_duplicate() does not require a lock on the parent
  chain, nor do we want to require one.  This can result in a race against
  indirect block creation which is trying to D-D chains from an upper chain
  to a lower one.

* Fix the problem by moving the DELETED check in
  hammer2_chain_create_indirect() from before the chain lock to after.

* Minor cleanup of the same routine.

* Add assertions to try to catch any situation where the block table is
  flushed twice, which would cause a later assertion.  Try to catch it
  earlier.