hammer2 - Implement meta-data statistics rollup

hammer2 - Implement meta-data statistics rollup

* HAMMER2 keeps total recursive data and inode count statistics in each
  inode.  This means that one can determine how much storage is being
  used for an entire subdirectory tree simply by doing a 'hammer2 stat <dir>'.

* Implement this by storing temporary rollup adjustments in the hammer2_chain
  structure, then synchronizing those adjustments on insertions, deletions,
  and flushes.

  Generally speaking, the chain structure has a data_count, inode_count,
  data_count_up, and inode_count_up for temporary tracking.  The main count
  fields are applied to the current chain AND the parent, while the *_up
  fields are only applied to the parent.

  For example, when an inode is inserted its stored statistics must be
  applied to the parent (recursively), but not to itself.

* Preliminary implementation.