A race in g_part_wither() can lead to I/O being performed with a freed GEOM
when the device disappears. Close the race as best as we can for now,
following the code patterns from g_part_ctl_destroy() and g_part_ctl_undo().
This also fixes a leak, as g_wither_geom() does not wither providers, it
only orphans them, so the partition entries would never get destroyed in
g_wither_washer().
Note, this is not a complete fix, it can still race with g_part_start(), the
race has merely been narrowed.
Reviewed by: markj
Sponsored by: Dell EMC Isilon
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