rtnl_unregister_ops calls __rtnl_links_kill which calls
unconditionally ops->dellink on any related interface.

However, if a module hasn't implemented dellink, this
invocation will crash.

Add a check to avoid calling dellink when NULL and rather
invoke netdevice_unregister_queue to schedule unregistering
the device as usual.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
---
 net/core/rtnetlink.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index f0a520987085..7e7fcd319af8 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c

@@ -453,8 +453,12 @@ static void __rtnl_kill_links(struct net *net, struct rtnl_link_ops *ops)
 	LIST_HEAD(list_kill);
 
 	for_each_netdev(net, dev) {
-		if (dev->rtnl_link_ops == ops)
-			ops->dellink(dev, &list_kill);
+		if (dev->rtnl_link_ops == ops) {
+			if (ops->dellink)
+				ops->dellink(dev, &list_kill);
+			else
+				unregister_netdevice_queue(dev, &list_kill);
+		}
 	}
 	unregister_netdevice_many(&list_kill);
 }

-- 
2.44.2