Legality
GPL2 only covers distribution, not usage. This means it is perfectly legal to download the source of ZFS, and the Linux kernel, and compile them together for use on your machine. It would only become illegal if you were to then distribute the resulting binary work - and even then only if the ZFS kernel module could be judged to be a 'derivative work' of the kernel under copyright law (very much an open question at this point, as it was originally implemented on the Solaris kernel).