kernel - cycle vnode in cache_unlink()

kernel - cycle vnode in cache_unlink()

* cache_unlink() and cache_rename() are called by VFS's to update the
  namecache after a file deletion or rename.  cache_rename() also calls
  cache_unlink().

* Add code to this function which cycles any vnode attached to the
  namecache entry for the case where its refs are 0.  This will
  trigger VOP_INACTIVE and allow the VFS to detect file deletions.

  Generally speaking this means that VFS's no longer have to detect
  deleted files in code paths outside of VOP_INACTIVE w/regards to
  forcing a reclaim on the vnode.

* HAMMER2 now depends on this behavior.  The change in behavior allows
  HAMMER2 to do away with inode backpointers from chain structures,
  greatly simplifying the code and removing a chicken-egg dependency.

* Other VFSs (HAMMER, TMPFS, UFS) may be adjusted in the future but for
  now their file deletion code outside of VOP_INACTIVE is being left
  intact.