hammer2 - add fifo/dev support, bug fixes

* Add vops for fifo, blk, and chr devices

* Fix bug in hammer2_chain_insert() - allow insertion races to push a new
  layer in all cases except when requested not to.

* Fix bug in hammer2_chain_duplicate() - Must call hammer2_chain_create()
  instead of hammer2_chain_insert() in case the duplication target needs
  indirect blocks.