MFC r298814 (by asomers): Fix a use-after-free when "zpool import" fails

clear vd->vdev_tsd in vdev_geom_close_locked instead of vdev_geom_detach.
In the latter function, it would fail to happen in certain circumstances
where cp->private was unset. Ideally, the latter should never happen, but
it can happen when vdev open fails, or where spares are involved.