https://blog.tastatursport.de/2016/05/zfs-mount-stuck-on-boot-ubuntu-16-04-xenial-xerus/

Fire up a LiveISO so you can manipulate files on the root system and edit the following:

sed -i -e 's/ZFS_MOUNT = 'yes'/ZFS_MOUNT = 'no'/g' /mnt/etc/default/zfs
sed -i -e 's/ZFS_SHARE = 'yes'/ZFS_SHARE = 'no'/g' /mnt/etc/default/zfs

When done, reenable via:

sed -i -e 's/ZFS_MOUNT = 'no'/ZFS_MOUNT = 'yes'/g' /etc/default/zfs
sed -i -e 's/ZFS_SHARE = 'no'/ZFS_SHARE = 'yes'/g' /etc/default/zfs