hammer2 - Flush sequencing

hammer2 - Flush sequencing

* Add a flag HAMMER2_CHAIN_DIRTYBP to differentiate between clean and
  dirty buffers, separate from HAMMER2_CHAIN_MODIFIED1.  We needed a
  new flag because a buffer can be dirtied and then written out by the
  OS (becoming clean) before the chain hierarchy itself is flushed.

  This fixes an issue where a small percentage of buffers were being
  written out several times without any new changes.

* Clean up HAMMER2_CHAIN_IOFLUSH and implement to reduce double buffering
  overheads, at least for the buffer cache.  The VM backing store might
  still have double-buffered data.

  This reduces the load on the buffer management code in the kernel and
  helps avoid potential deadlocks.

* Unlock the inode when stalling in bwillwrite().  This solves a problem
  where a program stuck in flstik can also stall out the buffer daemons.