zfsctl_snapdir_lookup should be able to handle an uncovered vnode

The uncovered vnode is possible because there is no guarantee that
its hold count would go to zero (and it would be inactivated and reclaimed)
immediately after a covering filesystem is unmounted.
So, such a vnode should be expected and it is possible to re-use it
without any trouble.

MFC after: 3 weeks
Sponsored by: Panzura