netns: restore ops before calling ops_exit_list

ops has been iterated to first element when call pre_exit, and
it needs to restore from save_ops, not save ops to save_ops

Fixes: d7d99872 ("netns: add pre_exit method to struct pernet_operations")
Signed-off-by: NLi RongQing <lirongqing@baidu.com>
Reviewed-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/net_namespace.c

@@ -345,7 +345,7 @@ static __net_init int setup_net(struct net *net, struct user_namespace *user_ns)
 
 	synchronize_rcu();
 
-	saved_ops = ops;
+	ops = saved_ops;
 	list_for_each_entry_continue_reverse(ops, &pernet_list, list)
 		ops_exit_list(ops, &net_exit_list);