hammer2 - correct saveoff in hammer2_vop_readdir()

hammer2 - correct saveoff in hammer2_vop_readdir()

* When terminating a search early read the next entry and set save_off
  to that, instead of setting it to the last returned entry's key + 1.

  This ensures an exact match when the search resume.  This could create
  problems in the future but for now its a quick way to track the
  directory scan due to the fully-associative nature of the on-media
  blockref arrays.

  (In the future we may have to sort the fully associative array on-media
  to really solve the issue).

* This corrects repeated entries in 'ls' output.