Fix for IPoIB over lagg(4).

Need to update both link layer address and broadcast address when active link changes for IP over infiniband.
This is because the broadcast address contains the so-called P-key, which is interface dependent.

Reviewed by: kib @
Differential Revision: https://reviews.freebsd.org/D27658
MFC after: 1 week
Sponsored by: Mellanox Technologies // NVIDIA Networking