some zfs changes for testing and review

[Andriy Gapon]

Could you please test and/or review some ZFS-related changes that can be found
in zfs-geom, zfs-vfs and zfs-vm branches that can be accessed here:

https://github.com/avg-I/freebsd/branches

The repo can be cloned using this URL: git://github.com/avg-I/freebsd.git

The branches are based off the FreeBSD head and I am merging the head to them
from time to time.
-- 
Andriy Gapon