The license incompatibility is on the GPL side, not on the ZFS / closed GPU side. The only ones which could sue are the Linux kernel developers.
Publish the source code files only under GPL and Linux kernel developers will be happy.
You might get intro trouble with the ZFS side however, since the CDDL license says: "must also be made available in Source Code form and that Source Code form must be distributed only under the terms of this License".
If the Source Code form is only under the terms of the GPL, then the above condition is not met. The law suit would thus come from the ZFS side.
Which includes Oracle, so they could sue Canonical and/or zfs users if I understand everything correctly
Doesn't Oracle provide a version of ZFS on its Linux?
ZFS on the other hand is owned by Oracle which has no interest in people using linux, nor ZFS if it's not on their proprietary stuff.