zfs: set acltype=posix for root-dataset

zfs: set acltype=posix for root-dataset

journald as a core component tries setting a ACL on the journal files
for (non-root) users and fails on our ZFS installs.
Resulting in dmesg being spammed with messages from journald upon each
journal-rotation for each user upon their first login.

This is also suggested by OpenZFS in their Debian guide for root on
ZFS:
https://openzfs.github.io/openzfs-docs/Getting%20Started/Debian/Debian%20Bookworm%20Root%20on%20ZFS.html

Tested by setting this on a machine of mine, where this has been
bugging for quite a while.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>