(kext) Avoid reusing a soft-revoked vnode

There are two flavors for revoking vnodes, soft and hard.

* In case of a hard revoke the vnode is reclaimed immediately. As a
  result the vnode is removed from our vnode cache.

* In case of a soft revoke the vnode is only maked as revoked, but not
  reclaimed immediately. This means the vnode stays in the vnode cache
  until all open file handles are closed.

In case a file disappears "behind our back" the corresponding vnode is
soft-revoked. If a new file appears with the same inode number we need
to make sure not to reuse the revoked vnode.