If available, record the physical path of a vdev in ZFS meta-data.
Do this both when opening the vdev, and when receiving an attribute
change notification from GEOM.

Make vdev_geom_close() synchronous instead of deferring its work to
a GEOM event handler. There is no benefit to deferring the work and
this prevents a future open call from referencing a consumer that is
scheduled for destruction. The close followed by an immediate open
will occur during a vdev reprobe triggered by any type of I/O error.

Consolidate vdev_geom_close() and vdev_geom_detach() into
vdev_geom_close() and vdev_geom_close_locked(). This also moves the
cross linking operations between vdev and GEOM consumer into a
single place (linking in vdev_geom_attach() and unlinking in
vdev_geom_close_locked()).