core: prevent spurious retries of umount

core: prevent spurious retries of umount

Testing the previous commit with `systemctl stop tmp.mount` logged the
reason for failure as expected, but unexpectedly the message was repeated
32 times.

The retry is a special case for umount; it is only supposed to cover the
case where the umount command was _successful_, but there was still some
remaining mount(s) underneath.  Fix it by making sure to test the first
condition :).

Re-tested with and without a preceding `mount --bind /mnt /tmp`,
and using `findmnt` to check the end result.