util-linux: util-linux: failed external mount helper still runs privileged X-mount post-hooks

When an external mount. helper runs but exits with a nonzero status, libmount still treats the helper invocation as successful and runs privileged post-mount hooks. A local unprivileged user with an /etc/fstab entry that uses the user option together with X-mount.idmap or X-mount.owner/group/mode...