After kernel upgrade, ZFS couldn’t start, but some process create some file in the mount point. So after that ZFS can’t start the process ever again. Here’s how to fix.
1. disable service zfs.target and restart.
user@ubuntu:~$ sudo systemctl disable zfs.target
2. after reboot delete or move data in mount point to another location.
3. enable service zfs.target and restart.
user@ubuntu:~$ sudo systemctl enable zfs.target
4. after reboot move data back.