In GEOM DISK:  - Replace single done mutex with per-disk ones.  On system with several disks on several HBAs that removes small, but measurable lock congestion.  - Modify disk destruction process to not destroy the mutex prematurely.  - Remove some extra pointer derefences.
In GEOM DISK:
 - Replace single done mutex with per-disk ones.  On system with several
disks on several HBAs that removes small, but measurable lock congestion.
 - Modify disk destruction process to not destroy the mutex prematurely.
 - Remove some extra pointer derefences.

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