hammer2 - stabilization

hammer2 - stabilization

* Fix more deadlocks.  The solution here is not ideal, I'm doing too much
  unlocking and relocking of clusters and probably breaking certain atomicy
  requirements.

* Implement a delayed flush mechanism when making permanent chain deletions
  instead of calling hammer2_flush().  This should allow the flush code to
  better optimize-out unnecessary write I/Os.

* Add a spinlock for the ip->cluster (not completed yet).

* I had broken system-initated filesystem syncing by calling vclrisdirty()
  on the syncer vnode.  Fix it.