There's no difference between the two types of filesystem; that's an arbitrary judgement. Both the GPL and CDDL are distribution licenses; using a ZFS module

There's no difference between the two types of filesystem; that's an arbitrary judgement. Both the GPL and CDDL are distribution licenses; using a ZFS module with Linux isn't a problem, you aren't violating any of the licences by using them together on your computer system. There's plenty of precedent for non-GPL kernel modules. It's not a problem in practice.