xfrm: set dst dev to blackhole_netdev instead of loopback_dev in ifdown

The global blackhole_netdev has replaced pernet loopback_dev to become the
one given to the object that holds an netdev when ifdown in many places of
ipv4 and ipv6 since commit 8d7017fd62 ("blackhole_netdev: use
blackhole_netdev to invalidate dst entries").

Especially after commit faab39f63c ("net: allow out-of-order netdev
unregistration"), it's no longer safe to use loopback_dev that may be
freed before other netdev.

This patch is to set dst dev to blackhole_netdev instead of loopback_dev
in ifdown.

v1->v2:
  - add Fixes tag as Eric suggested.

Fixes: faab39f63c ("net: allow out-of-order netdev unregistration")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/e8c87482998ca6fcdab214f5a9d582899ec0c648.1652665047.git.lucien.xin@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/xfrm/xfrm_policy.c

@@ -3744,7 +3744,7 @@ static int stale_bundle(struct dst_entry *dst)
 void xfrm_dst_ifdown(struct dst_entry *dst, struct net_device *dev)
 {
 	while ((dst = xfrm_dst_child(dst)) && dst->xfrm && dst->dev == dev) {
-		dst->dev = dev_net(dev)->loopback_dev;
+		dst->dev = blackhole_netdev;
 		dev_hold(dst->dev);
 		dev_put(dev);
 	}