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: 4477138f ("tun: properly test for IFF_UP")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: Nsyzbot <syzkaller@googlegroups.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1960,6 +1960,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 	rcu_read_lock();
 	if (unlikely(!(tun->dev->flags & IFF_UP))) {
 		err = -EIO;
+		rcu_read_unlock();
 		goto drop;
 	}