hammer2 - file truncate and extend

hammer2 - file truncate and extend

* Implement file truncation and extension.  Remember that all partial block
  modifications (during truncation) have to roll a new block, so we use
  the chain_modify and related functions to handle it.

* Implement inode-embedded direct data (up to 512 bytes)

* Add support for the embedded feature in the chain_lookup/chain_next
  code and also fix a couple of bugs.

* Implement deletion flagging for chain structures (used for truncation and
  will eventually be used for unlink).

* Initial implementation of setattr (for truncation support).