hammer2 - Get data-usage aggregation working, add INODE_GET

hammer2 - Get data-usage aggregation working, add INODE_GET

* Cleanup aggregation of data_count and inode_count in the inode.
  data_count should now work properly (though it requires a 'sync'
  if you want up-to-date information).

  This allow data and inode usage for an entire sub-tree to be
  retrieved from the parent directory inode.  No need to run 'du'
  over millions of inodes.

  The new 'hammer2 stat' command can be used to access the info.

* Add the HAMMER2IOC_INODE_GET/SET ioctls to access information that
  cannot be obtained from a normal stat().