Make GEOM RAID more aggressive in marking volumes as clean on shutdown and move that action from shutdown_pre_sync to shutdown_post_sync stage to avoid extra flapping.
Make GEOM RAID more aggressive in marking volumes as clean on shutdown
and move that action from shutdown_pre_sync to shutdown_post_sync stage
to avoid extra flapping.

ZFS tends to not close devices on shutdown, that doesn't allow GEOM RAID
to shutdown gracefully.  To handle that, mark volume as clean just when
shutdown time comes and there are no active writes.

MFC after:	2 weeks

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