bgpd: reset ipv6 invalid link-local nexthop

bgpd: reset ipv6 invalid link-local nexthop

If the "nexthop-local unchanged" setting is enabled, it preserves the
IPv6 link-local nexthop from the originating peer. However, if the
originating and destination peers are not on the same network segment,
the originating peer's IPv6 link-local address will be unreachable from
the destination peer.

In such cases, reset the IPv6 link-local nexthop, even if "nexthop-local
unchanged" is set on the destination peer.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>
(cherry picked from commit 5bb99ccad2935adb006f7f727c2779b2c6e746e5)