ZFS legacy mount fails

[Илья Валеев <ilya-valeev@yandex.ru>]

Hello!

I'm trying to mount ZFS dataset with /etc/fstab file. Added string in it:

doublediskpool/systemlog /mnt zfs relatime,defaults 0 2

Rebooted, and system fails to boot. In boot log I see that spl and zfs
modules loaded, than:

systemd[1]: mnt.mount: Mount process exited, code=exited status=1
systemd[1]: Failes to mount /mnt.

After this zfs.target successfully starts, and system fall into
emergency mode.
Tryed to add in [Install] section of /lib/systemd/system/zfs.target such
string:

Before=local-fs.target

but it does not help.
If I enter command (that systemd fail to execute) in emergency mode or
booted system:

# mount doublediskpool/systemlog /mnt -t zfs -o relatime,defaults

than mount executed successfully.

Nothing changes if I set another mountpoint.

How this can be fixed?
Debian 9 with last updates.
Thanks!