netlink: Don't directly access ifnet members

Summary:
Remove the final direct access of struct ifnet members from netlink.
Since only the first address is used, create the iterator and then free,
without fully iterating.

Reviewed By: kp
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D42972

(cherry picked from commit b224af946a17b8e7a7b4942157556b5bc86dd6fb)