dn_route: set rt neigh to blackhole_netdev instead of loopback_dev in ifdown

Like other places in ipv4/6 dst ifdown, change to use blackhole_netdev
instead of pernet loopback_dev in dn dst ifdown.
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/0cdf10e5a4af509024f08644919121fb71645bc2.1652751029.git.lucien.xin@gmail.comSigned-off-by: NJakub Kicinski <kuba@kernel.org>

net/decnet/dn_route.c

@@ -159,7 +159,7 @@ static void dn_dst_ifdown(struct dst_entry *dst, struct net_device *dev, int how
 		struct neighbour *n = rt->n;
 
 		if (n && n->dev == dev) {
-			n->dev = dev_net(dev)->loopback_dev;
+			n->dev = blackhole_netdev;
 			dev_hold(n->dev);
 			dev_put(dev);
 		}