makefs: correct iso9660 Rock Ridge NM values

These are not actually used by makefs (yet), but ought to match the
spec.

See RRIP 4.1.4 Description of the "NM" System Use Entry for details.

PR: 203531
Sponsored by: The FreeBSD Foundation

(cherry picked from commit c3ec19359e05c3b609e461896aa6ab4d6ad8121f)