Which is still CDDL licensed and thus GPLv2 incompatible.

Here's the copyright file from the relevant commit to the Xenial kernel tree:

http://kernel.ubuntu.com/git/ubuntu/ubuntu-xenial.git/diff/z...

    The majority of the code in the ZFS on Linux port comes from
    OpenSolaris which has been released under the terms of the CDDL
    open source license.  This includes the core ZFS code, libavl,
    libnvpair, libefi, libunicode, and libutil.

OpenZFS was forked from the original Sun/Solaris ZFS code base, which Sun made available regularly. It's still CDDL.