tun: fix a memory leak for tfile->tx_array

tfile->tun could be detached before we close the tun fd,
via tun_detach_all(), so it should not be used to check for
tfile->tx_array.

As Jason suggested, we probably have to clean it up
unconditionally both in __tun_deatch() and tun_detach_all(),
but this requires to check if it is initialized or not.
Currently skb_array_cleanup() doesn't have such a check,
so I check it in the caller and introduce a helper function,
it is a bit ugly but we can always improve it in net-next.

Reported-by: Dmitry Vyukov <[e-mail]>
Fixes: 1576d9860599 ("tun: switch to use skb array for tx")
Cc: Jason Wang <[e-mail]>
Signed-off-by: Cong Wang <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/tun.c

···
 	skb_queue_purge(&tfile->sk.sk_error_queue);
 }
 
+static void tun_cleanup_tx_array(struct tun_file *tfile)
+{
+	if (tfile->tx_array.ring.queue) {
+		skb_array_cleanup(&tfile->tx_array);
+		memset(&tfile->tx_array, 0, sizeof(tfile->tx_array));
+	}
+}
+
 static void __tun_detach(struct tun_file *tfile, bool clean)
 {
 	struct tun_file *ntfile;
···
 			    tun->dev->reg_state == NETREG_REGISTERED)
 				unregister_netdevice(tun->dev);
 		}
-		if (tun)
-			skb_array_cleanup(&tfile->tx_array);
+		tun_cleanup_tx_array(tfile);
 		sock_put(&tfile->sk);
 	}
 }
···
 		/* Drop read queue */
 		tun_queue_purge(tfile);
 		sock_put(&tfile->sk);
+		tun_cleanup_tx_array(tfile);
 	}
 	list_for_each_entry_safe(tfile, tmp, &tun->disabled, next) {
 		tun_enable_queue(tfile);
 		tun_queue_purge(tfile);
 		sock_put(&tfile->sk);
+		tun_cleanup_tx_array(tfile);
 	}
 	BUG_ON(tun->numdisabled != 0);
 
···
 	INIT_LIST_HEAD(&tfile->next);
 
 	sock_set_flag(&tfile->sk, SOCK_ZEROCOPY);
+
+	memset(&tfile->tx_array, 0, sizeof(tfile->tx_array));
 
 	return 0;
 }