hammer2 - stabilization

* Reduce HAMMER2_FLUSH_DEPTH_LIMIT from 40 to 10 to avoid blowing out
  the kernel stack.

* Retool hammer2_chain_drop() and hammer2_chain_lastdrop() to remove all
  possible recursion.  The in-memory topology can get very deep and very
  wide.  This fixes another kernel stack blowout.

* Fix a bug in hammer2_chain_flush()'s deferred flush.  Now that
  hammer2_chain_flush() can replace the passed-in chain, we have to drop
  the extra ref before calling it instead of after.