When adding stop command to the serialization queue, if the command is
getting added to the pending queue of the vdev and there already existed
another stop command in the same queue, then return already exists as the
return value after the command is enqueued.
This to avoid notifying the vdev state machine with down event multiple
times though the command already in the pending queue would have notified.
Change-Id: I1830251c6b6c1fa2860f17a3ea8a869e8a3c87ff
CRs-Fixed: 2372563
ba57c33317