mountd(8): parsecred(): uid:gid:... loop: Simplify a bit

No functional change intended.

Reviewed by: rmacklem (older version)
Approved by: markj (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46919

(cherry picked from commit ae22a4bb7437019e34fc593e101a6ac14c9d7959)