MAC/do: Rename internal mac_do_rule_find() => find_rules()

To simplify, be consistent with the rename 'struct mac_do_rule' =>
'struct rules' and other functions, and because this function is
internal (and thus is never the first mac_do(4)'s function to appear in
a stack trace).

Reviewed by: bapt
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47594