ipv4: Honor "ignore_routes_with_linkdown" sysctl in nexthop selection

Commit 32607a332cfe ("ipv4: prefer multipath nexthop that matches source
address") changed IPv4 nexthop selection to prefer a nexthop whose
nexthop device is assigned the specified source address for locally
generated traffic.

While the selection honors the "fib_multipath_use_neigh" sysctl and will
not choose a nexthop with an invalid neighbour, it does not honor the
"ignore_routes_with_linkdown" sysctl and can choose a nexthop without a
carrier:

$ sysctl net.ipv4.conf.all.ignore_routes_with_linkdown
net.ipv4.conf.all.ignore_routes_with_linkdown = 1
$ ip route show 198.51.100.0/24
198.51.100.0/24
nexthop via 192.0.2.2 dev dummy1 weight 1
nexthop via 192.0.2.18 dev dummy2 weight 1 dead linkdown
$ ip route get 198.51.100.1 from 192.0.2.17
198.51.100.1 from 192.0.2.17 via 192.0.2.18 dev dummy2 uid 0

Solve this by skipping over nexthops whose assigned hash upper bound is
minus one, which is the value assigned to nexthops that do not have a
carrier when the "ignore_routes_with_linkdown" sysctl is set.

In practice, this probably does not matter a lot as the initial route
lookup for the source address would not choose a nexthop that does not
have a carrier in the first place, but the change does make the code
clearer.

Signed-off-by: Ido Schimmel <[e-mail]>
Reviewed-by: Willem de Bruijn <[e-mail]>
Reviewed-by: David Ahern <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/fib_semantics.c

···
 	saddr = fl4 ? fl4->saddr : 0;
 
 	change_nexthops(fi) {
-		if (use_neigh && !fib_good_nh(nexthop_nh))
+		int nh_upper_bound;
+
+		/* Nexthops without a carrier are assigned an upper bound of
+		 * minus one when "ignore_routes_with_linkdown" is set.
+		 */
+		nh_upper_bound = atomic_read(&nexthop_nh->fib_nh_upper_bound);
+		if (nh_upper_bound == -1 ||
+		    (use_neigh && !fib_good_nh(nexthop_nh)))
 			continue;
 
 		if (!found) {
···
 			found = !saddr || nexthop_nh->nh_saddr == saddr;
 		}
 
-		if (hash > atomic_read(&nexthop_nh->fib_nh_upper_bound))
+		if (hash > nh_upper_bound)
 			continue;
 
 		if (!saddr || nexthop_nh->nh_saddr == saddr) {