Rework syncer termination code:
Rework syncer termination code:

    Speed up the syncer when shutting down by sleeping for a shorter
    period of time instead of cranking up rushjob and using the
    normal one second sleep.

    Skip empty worklist slots when shutting down to avoid lengthy
    intervals of inactivity.

    Give I/O more time to complete between steps by not speeding the
    syncer quite as much.

    Terminate the syncer after one full pass through the worklist
    plus one second with the worklist containing nothing but syncer
    vnodes.

    Print an indication of shutdown progress to the console.

Add a sysctl, vfs.worklist_len, to allow the size of the syncer worklist
to be monitored.

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