geom_slice: do not destroy softc until providers are gone
At present, g_slice_orphan and g_slice_spoiled destroy the softc
(struct g_slicer) even before calling g_wither_geom, so there can
be active and incoming io requests at that time and g_slice_start
can access the softc.
This commit changes the code to destroy the softc only after all
providers are closed.