makefs: Crudely fix a sprintf warning

Reviewed by: kevans
Obtained from: OpenBSD d95291fdb2dc
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48178

(cherry picked from commit 2069f6ecb1b1268deac980176c652ffab264970e)
(cherry picked from commit fb0239e69f17400b08421a002a72b9520afd8285)