OpenZFS is still open source and available on FreeBSD (amongst others). The licence might not be GPL compatible but the way ZFS is shipped on Linux there isn't any GPL/CDDL mixed code bases (and it's not like there isn't already a long history of GPL-incompatible software licences with Linux kernel modules anyway).
Hence, Canonical (or anyone else trying to re-release under the CDDL) cannot distribute binaries of the kernel modules.
However, you are fully allowed to compile and link the two on your own. This is why the alternative to binary distribution is compiling yourself with the help of DKMS.
As a result, it would be extraordinarily difficult for Oracle to sue since it would not be the CDDL licence terms which were infringed.
That said, boomboomsubban (https://news.ycombinator.com/item?id=24331590) suggested Oracle could sue on the GPL side of things since they have committed code to Linux. It would be interesting if such a position is possible and I suspect that could open the door to all sorts of frivolousness cases (eg closed binaries shipped with Android).