> So, that would seem to leave two options - either re-license Linux under a CDDL-compatible license or re-license ZFS under a GPL-compatible license. Clearly re-licensing ZFS would be much easier as it's controlled by a single entity - Oracle.

Oracle's ZFS code is controlled by a single entity, but OpenZFS (which is what you probably want) is a community fork with copyrights owned by a huge list of people and companies.