[patch] zfs livelock and thread priorities

[Artem Belevich]

> I've updated the patch to use va_copy().  Unfortunately I don't have an
> AMD64 machine to test on at the moment.  However, I did look at other places
> in the kernel where va_copy() is used so I am hopeful I got it right.

The latest diff does not crash on amd64. That's the good news.

Even better news is that the hang that I had with enabled ZIL under
heavy load now seems to be gone.
The build that was usually triggering the hang after about an hour has
been going for about three hours now.

Thanks a lot for the diff. It does help to bring ZFS on FreeBSD closer
to production quality.

--Artem