Remount datasets on soft-reboot

The one-shot zfs-mount.service is incorrectly deemed active by
Systemd after a systemctl soft-reboot. As such, soft-rebooting
prevents zfs mount -a from being ran automatically.

This commit makes it so that zfs-mount.service is marked as being
undone by the time umount.target is reached, so that zfs.target then
pulls it in again and gets it restarted after a soft reboot.

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: kotauskas <v.toncharov@gmail.com>
Closes #16845