Rollback before zfs root is mounted

Rollback before zfs root is mounted

On my machines I observe random failures caused by rollback happening
after zfs root is mounted. I've observed two types of failures:

- zfs-rollback-bootfs.service fails saying that rollback must be
  done just before mounting the dataset
- boot process fails and rescue console is entered.

After making this modification and testing it for couple of days
none of those problems have been observed anymore.

I don't know if `dracut-mount.service` is still needed in the
`After` directive. Maybe someone else is able to address this?

Reviewed-by: Gregory Bartholomew <gregory.lee.bartholomew@gmail.com>
Signed-off-by: Wojciech Małota-Wójcik <59281144+outofforest@users.noreply.github.com>
Closes #15025