VFS change and nullfs

[YONETANI Tomokazu <qhwt+dragonfly-bugs@xxxxxxxxxx>]

Hello.
It seems that nullfs still has an issue after vfs04.patch came in.
Previously, make -j100 buildworld on top of null-mounted /usr/obj
finished without a problem(I know some of you might say `nullfs never
worked right even on FreeBSD,' but it does at least on 4.x). Now it
gets stuck in the middle of buildworld, with a few processes in sitting
in vnode state, just as I reported to Matt when I tried vfs03.patch.
The current kernel proceeds much further than it did with vfs03.patch,
but it eventually stuck at somewhere and buildworld won't finish.

Besides that, after manually panicking the box to save the kernel core,
it keeps printing a message like this every several seconds

  CHKTOKEN loop 0
  CHKTOKEN loop 0

and wouldn't reboot until I pressed the reset button. After pressing the
reset button, savecore reported that the dump device didn't contain the
dump image, and I couldn't get the kernel core. I'm doing the buildworld
on Netfinity 6000R with 2 PentiumIII and SMP kernel(and I'm using an old
IDE disk to dump the kernel core).