An AC said: "Which could get Canonical into hot water with the GPL."
Whether or not this is a licence violation depends on Linus Torvalds and The Linux Foundation. They are the ones that set the terms for how Linux is licensed. Under U.S. law at least, it's the copyright owner's intent that matters, and not some third party interpretation interpretation of the licence text.
Torvalds has previously stated that a kernel module can't violate the kernel licence agreement unless it is a derivative work of the kernel (and the module licence violates the GPL). At the very least, it needs to have been designed with knowledge of the Linux internals. Since ZFS was developed independent of Linux, it seems unlikely that The Linux Foundation will be suing Canonical.
If you want to thoroughly understand the issues, you could read Eben Moglen's opinion (he's the lawyer behind the GPL 3): https://www.softwarefreedom.org/resources/2016/linux-kernel-cddl.html