net: track link status of ipv6 nexthops

Add support to track current link status of ipv6 nexthops to match
recent changes that added support for ipv4 nexthops.  This takes a
simple approach to track linkdown status for next-hops and simply
checks the dev for the dst entry and sets proper flags that to be used
in the netlink message.

v2: drop use of rt6i_nhflags since it is not needed right now

Signed-off-by: Andy Gospodarek <gospo@cumulusnetworks.com>
Signed-off-by: Dinesh Dutt <ddutt@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -2887,6 +2887,8 @@ static int rt6_fill_node(struct net *net,
 	else
 		rtm->rtm_type = RTN_UNICAST;
 	rtm->rtm_flags = 0;
+	if (!netif_carrier_ok(rt->dst.dev))
+		rtm->rtm_flags |= RTNH_F_LINKDOWN;
 	rtm->rtm_scope = RT_SCOPE_UNIVERSE;
 	rtm->rtm_protocol = rt->rt6i_protocol;
 	if (rt->rt6i_flags & RTF_DYNAMIC)