hammer2 - hardlink stabilization (3), data and inode count propagation.
* Files with cached chains have to be flushed before they can be copied
to the hardlink target, because the original inode will become a
OBJTYPE_HARDLINK pointer which isn't allowed to have any sub-chains
under the inode.
* We also need to flush for the upcoming snapshot function to work properly
or dirty in-memory data will not show up in the snapshot.
* Propagate the inode and byte use count up the chain. Tie the inode count
into df's inode count (per-PFS). The byte count and quota fields are not
yet tied in.
* Adjust stat[v]fs() to return filesystem space useage using the allocation
iterator for now, to aid debugging.
* Adjust the allocation iterator to skip reserved areas at the beginning of
each 2GB storage zone.