hammer2 - Add directive to destroy bad directory entries

hammer2 - Add directive to destroy bad directory entries

* Add a directive and ioctl that is capable of destroying bad hammer2
  directory entries.  If topological corruption occurs due to a crash
  (which theoretically shouldn't be possible with HAMMER2), this directive
  allows you to destroy directory entries which do not have working inodes
  and cannot otherwise be destroyed with 'rm'.

* Sysops should only use this directive when absolutely necessary.