MAC/do: find_rules(): Clarify the contract

While here, rename an internal variable.

Reviewed by: bapt
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47596

(cherry picked from commit b2c661fe7e0b0dff859767a6a8714198b38dc235)