MAC/do: Rename private struct 'mac_do_rule' => 'rules'

To simplify and be consistent with 'struct rule'.

Reviewed by: bapt
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47593

(cherry picked from commit 02ed945ccec43340208d3a9c152fb98f55dbed69)