zebra: read multipath routes and hw addr from netlink

zebra: read multipath routes and hw addr from netlink

  * zebra/rt_netlink.c:

    - Pick up the hardware address of an interface when we receive a
      netlink link change message. Extract code for parsing the
      link-layer hardware address into a new function so we can reuse
      it.

    - netlink_routing_table(): Update to handle multipath routes.

    - netlink_route_change(): Update to handle multipath routes. Fix
      problem where the metric was not being read out.

  * zebra/zebra_rib.[ch]: Extern nexthop_ipv4_ifindex_add() -- it is
    now called from the netlink code.

From: Josh Bailey <joshb@google.com>
Signed-off-by: Avneesh Sachdev <avneesh@opensourcerouting.org>
Signed-off-by: David Lamparter <equinox@diac24.net>