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

[olce]

Summary

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This series has also been squeezed into D47633 to provide an overall view.

Commit message:
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).

[bapt]

same about the commit message

[olce]

In D47594#1086927, @bapt wrote:

same about the commit message

Done.