tun: add a missing rcu_read_unlock() in error path

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

Fixes: 4477138fa0ae ("tun: properly test for IFF_UP")
Signed-off-by: Eric Dumazet <[e-mail]>
Reported-by: syzbot <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/tun.c

···
 	rcu_read_lock();
 	if (unlikely(!(tun->dev->flags & IFF_UP))) {
 		err = -EIO;
+		rcu_read_unlock();
 		goto drop;
 	}