net: net_namespace: Fix undefined member in key_remove_domain()

The key_domain member in struct net only exists if we define CONFIG_KEYS.
So we should add the define when we used key_domain.

Fixes: 9b242610 ("keys: Network namespace domain tag")
Signed-off-by: NYajun Deng <yajun.deng@linux.dev>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/net_namespace.c

@@ -473,7 +473,9 @@ struct net *copy_net_ns(unsigned long flags,
 
 	if (rv < 0) {
 put_userns:
+#ifdef CONFIG_KEYS
 		key_remove_domain(net->key_domain);
+#endif
 		put_user_ns(user_ns);
 		net_free(net);
 dec_ucounts:
@@ -605,7 +607,9 @@ static void cleanup_net(struct work_struct *work)
 	list_for_each_entry_safe(net, tmp, &net_exit_list, exit_list) {
 		list_del_init(&net->exit_list);
 		dec_net_namespaces(net->ucounts);
+#ifdef CONFIG_KEYS
 		key_remove_domain(net->key_domain);
+#endif
 		put_user_ns(net->user_ns);
 		net_free(net);
 	}