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.