tun: add a missing rcu_read_unlock() in error path

commit 9180bb4f04 upstream.

In my latest patch I missed one rcu_read_unlock(), in case
device is down.

Fixes: 4477138fa0 ("tun: properly test for IFF_UP")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/net/tun.c

@@ -1330,6 +1330,7 @@ drop:
 	rcu_read_lock();
 	if (unlikely(!(tun->dev->flags & IFF_UP))) {
 		err = -EIO;
+		rcu_read_unlock();
 		goto drop;
 	}