ipv6: Don't dev_hold(dev) in ip6_mc_find_dev_rcu.

ip6_mc_find_dev_rcu() is called with rcu_read_lock(), so don't
need to dev_hold().
With dev_hold(), not corresponding dev_put(), will lead to leak.

[ bug introduced in 96b52e61 (ipv6: mcast: RCU conversions) ]
Signed-off-by: RongQing.Li <roy.qing.li@gmail.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/mcast.c

@@ -257,7 +257,6 @@ static struct inet6_dev *ip6_mc_find_dev_rcu(struct net *net,
 
 		if (rt) {
 			dev = rt->dst.dev;
-			dev_hold(dev);
 			dst_release(&rt->dst);
 		}
 	} else