shutdown: Don't limit unmount attempts prematurely (#8469)

shutdown: Don't limit unmount attempts prematurely (#8469)

Once upon a time shutdown.c didn't have the logic to check whether any
unmount attempts succeeded or not. So instead it kept looping for
a fixed amount and hoped all was right. Nowadays, we do know if we
changed anything during a iteration and also stop looping then, but
we still limit ourselves to FINALIZE_ATTEMPTS.

But, theoretically, we could have such a complicated and nested
setup that would survive that limit, leaving stuff around we
might actually be able to unmount. And we could also end up in a
situation where the extra loop with raised unmount error level could
be skipped too.

So let's just drop the retries logic and rely fully on the changed
flag.