rules: allow systemd to manage loop device partitions

SYSTEMD_READY is currently set to 0 for all loop devices (loop[0-9]*)
that do not have a backing_file. Partitioned loop devices (ex. loop0p1),
however, are matched by this rule and excluded by systemd even though
they are active devices.

This change adds an additional check to the rule, ensuring that only
top level loop devices (loop[0-9]+$) are excluded from systemd.