I built a version for linux 5.5.8 for you from the latest master (https://github.com/openzfs/zfs/tree/7e3df9db128722143734a9459771365ea19c1c40) which includes that fix you refrenced.
You can find that build here
https://www.dropbox.com/s/i6cuvqnka3y64vs/zfs-0.8.3-unRAID-6.9.0-beta1.x86_64.tgz?dl=0
https://www.dropbox.com/s/6e64mb8j9ynokj8/zfs-0.8.3-unRAID-6.9.0-beta1.x86_64.tgz.md5?dl=0
Copy both these files and replace with the files in /boot/config/plugins/unRAID6-ZFS/packages/ and reboot
To verify that you are on the correct build type: dmesg | grep ZFS and you should see "ZFS: Loaded module v0.8.0-1," (the version name from master https://github.com/openzfs/zfs/blob/7e3df9db128722143734a9459771365ea19c1c40/META)
FYI only kernel versions up to 5.4 are officially supported according to the META file above.
Have fun