blackhole_netdev: use blackhole_netdev to invalidate dst entries

jira LE-3201
Rebuild_History Non-Buildable kernel-rt-4.18.0-553.27.1.rt7.368.el8_10
commit-author Mahesh Bandewar <maheshb@google.com>
commit 8d7017f

Use blackhole_netdev instead of 'lo' device with lower MTU when marking
dst "dead".

	Signed-off-by: Mahesh Bandewar <maheshb@google.com>
	Tested-by: Michael Chan <michael.chan@broadcom.com>
	Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 8d7017f)
	Signed-off-by: Jonathan Maple <jmaple@ciq.com>

Author: PlaidCat

net/core/dst.c

@@ -170,7 +170,7 @@ void dst_dev_put(struct dst_entry *dst)
 		dst->ops->ifdown(dst, dev, true);
 	dst->input = dst_discard;
 	dst->output = dst_discard_out;
-	dst->dev = dev_net(dst->dev)->loopback_dev;
+	dst->dev = blackhole_netdev;
 	dev_hold(dst->dev);
 	dev_put(dev);
 }

net/ipv4/route.c

@@ -1570,7 +1570,6 @@ static void ipv4_dst_destroy(struct dst_entry *dst)
 
 void rt_flush_dev(struct net_device *dev)
 {
-	struct net *net = dev_net(dev);
 	struct rtable *rt;
 	int cpu;
 
@@ -1581,7 +1580,7 @@ void rt_flush_dev(struct net_device *dev)
 		list_for_each_entry(rt, &ul->head, rt_uncached) {
 			if (rt->dst.dev != dev)
 				continue;
-			rt->dst.dev = net->loopback_dev;
+			rt->dst.dev = blackhole_netdev;
 			dev_hold(rt->dst.dev);
 			dev_put(dev);
 		}

net/ipv6/route.c

@@ -183,7 +183,7 @@ static void rt6_uncached_list_flush_dev(struct net *net, struct net_device *dev)
 			}
 
 			if (rt_dev == dev) {
-				rt->dst.dev = loopback_dev;
+				rt->dst.dev = blackhole_netdev;
 				dev_hold(rt->dst.dev);
 				dev_put(rt_dev);
 			}