xvrf: simplify VRF interface retrieval

ip_output/ip6_output already performs:
    iface = iface_from_id(nh->iface_id);
    mbuf_data(mbuf)->iface = iface;

xvrf_loop_process was doing this again unnecessarily. Remove the
redundant code.

TODO: verify multicast IPv6. (should never happens ?)

Signed-off-by: Maxime Leroy <maxime@leroys.fr>

Author: maxime-leroy