MAC/do: Prefix internal functions used as hooks/callbacks

So that we immediately know whether a kernel stack involves MAC/do.

Reviewed by: bapt
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47607

(cherry picked from commit 11ba1f2fe2d4e151ffc0a66d03a0691a7b8d2866)