ZFS is not part of linux. It is a separate project that publishes both to FreeBSD and some shims so you can run it on Linux. FreeBSD pulls and directly integrates it, but for license reasons linux doesn't pull it in directly. Yes the upstream project does have some shims of linux use that they public directly, but FreeBSD is a first class user of that project while the linux shims are a hack that some in the kernel do not want to work.