acl_equiv_mode_np: zero mode on just inited ACL

You can't return 0 and not write the mode if mode_p is non-NULL. That
violates the API contract and in common usage leaves stack trash in
*mode_p.

The acl_equiv_mode_test test passed by accident.

Reviewed by: kevans, markj
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D43278