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: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>