core: systemd-shutdown: add missing check for umount_changed

core: systemd-shutdown: add missing check for umount_changed

The assumption was that nothing changes in the final attempt.  This
would be confusing if a filesystem with a process in uninterruptible
sleep suddenly became un-stuck for the final attempt, but we still give
up and don't try to e.g. unmount any parent mounts.

I don't know how possible that is.  But the code will be easier to read
without an assumption that it does not attempt to justify.