[lltable] Add per-family lltable getters.

Introduce a new function, lltable_get(), to retrieve lltable pointer
for the specified interface and family.
Use it to avoid all-iftable list traversal when adding or deleting
ARP/ND records.

Differential Revision: https://reviews.freebsd.org/D33660
MFC after: 2 weeks

(cherry picked from commit ff3a85d32411cdd7894f932b1d3d7ce01ec7a648)