The version of ZFS that everybody (besides the dwindling number of Oracle Solaris customers) uses now, OpenZFS, has been maintained completely independently of Oracle since they shut down OpenSolaris in 2010. This means that Oracle relicensing ZFS wouldn't do anything to help with getting it integrated into the Linux kernel, since there's been hundreds of independent contributors to ZFS since then who all own their own copyrights. Because ZFS is licensed under the CDDL, which has an automatic upgrade clause, Oracle could simply copy/paste the GPLv2 license text and call it "CDDL v2" if they wanted to make ZFS able to be included in Linux.