ipv6: Do not consider linkdown nexthops during multipath

BugLink: http://bugs.launchpad.net/bugs/1738219

When the 'ignore_routes_with_linkdown' sysctl is set, we should not
consider linkdown nexthops during route lookup.

While the code correctly verifies that the initially selected route
('match') has a carrier, it does not perform the same check in the
subsequent multipath selection, resulting in a potential packet loss.

In case the chosen route does not have a carrier and the sysctl is set,
choose the initially selected route.

Fixes: 35103d11 ("net: ipv6 sysctl option to ignore routes when nexthop link is down")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: David Ahern <dsahern@gmail.com>
Acked-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit bbfcd776)
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

net/ipv6/route.c

@@ -465,6 +465,11 @@ static struct rt6_info *rt6_multipath_select(struct rt6_info *match,
 				&match->rt6i_siblings, rt6i_siblings) {
 			route_choosen--;
 			if (route_choosen == 0) {
+				struct inet6_dev *idev = sibling->rt6i_idev;
+
+				if (!netif_carrier_ok(sibling->dst.dev) &&
+				    idev->cnf.ignore_routes_with_linkdown)
+					break;
 				if (rt6_score_route(sibling, oif, strict) < 0)
 					break;
 				match = sibling;