Json

Note: not necessarily indending to push this to zfsonlinux.  Creating a pull request seems to be the only way of leaving code review comments?  Let me know if I'm using github wrong.
did you check all callbacks used with zfs_for_each() to make sure that they handle (and ignore, presumably) invocations with a NULL first argument?

If possible, it would be cleaner to remove this, and instead have the caller do whatever it needs after calling zfs_for_each().