IfAPI: Hide the in6m_lookup_locked() implementation.

Summary:
in6m_lookup_locked() iterates over the ifnet's multiaddrs list. Keep
this implementation detail private, by moving the implementation to the
netstack source from the header.

Sponsored by: Juniper Networks, Inc.
Reviewed by: glebius, melifaro
Differential Revision: https://reviews.freebsd.org/D38201