[issue2039] Sometimes, DragonFly 2.9 systems can not reboot

[Matthew Dillon]

    I've figured it out.  I looked at the core Francois Tigeot provided
    (sorry if others were provided before, it was on my list!)... in
    anycase, there is a bug in the HAMMER flusher which can cause it
    to loose track of the flush sequence number which umountall can
    trigger due to the extra flushes hammer does on unmount.

    Commit e86903d84f840af38d1b452a6a6c624702373751 should fix it.

						-Matt