re-tasting of providers held with withering consumers

[Andriy Gapon]

Because removal of withered geoms is done asynchronously, there is a window when
some provider may require re-tasting (because of media change or size change),
but it would still be in use by the withering geom.  That prevents re-tasting a
class of that withering geom (for obvious reasons).

The following patch tries to trigger owed re-tasting after the withering
provider is gone for good:
http://people.freebsd.org/~avg/geom-withered-retaste.diff

Probably the retasting should be more targeted similarly to what g_retaste does,
but in asynchronous mode.  This is a missing piece.

-- 
Andriy Gapon