hammer2 - Add truncation lock, change dio persistence

hammer2 - Add truncation lock, change dio persistence

* Add a truncation lock to interlock between write()'s and ftruncate()
  calls.  This prevents a junk buffer from surviving a ftruncate() if
  it happens to get written concurrently with the ftruncate().

* Change dio persistence.  Do not automatically persist dio's while refs
  are held, this creates problems for ref'd chains held in the
  hammer2_inode_t structure and prevents vfsync() from working properly.

  Add a persist_refs field which we will use later in the XIO code to
  persist a chain's DIO across an unlock/relock sequence, and possibly
  in other places too.