bridge: fix rcu dereference outside of rcu_read_lock

Alternative solution for problem found by Linux Driver Verification
project (linuxtesting.org).

As it noted in the comment before the br_handle_frame_finish
function, this function should be called under rcu_read_lock.

The problem callgraph:
br_dev_xmit -> br_nf_pre_routing_finish_bridge_slow ->
 -> br_handle_frame_finish -> br_port_get_rcu -> rcu_dereference

And in this case there is no read-lock section.
Reported-by: NDenis Efremov <yefremov.denis@gmail.com>
Signed-off-by: NStephen Hemminger <shemminger@vyatta.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/bridge/br_device.c

@@ -31,9 +31,11 @@ netdev_tx_t br_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 	struct net_bridge_mdb_entry *mdst;
 	struct br_cpu_netstats *brstats = this_cpu_ptr(br->stats);
 
+	rcu_read_lock();
 #ifdef CONFIG_BRIDGE_NETFILTER
 	if (skb->nf_bridge && (skb->nf_bridge->mask & BRNF_BRIDGED_DNAT)) {
 		br_nf_pre_routing_finish_bridge_slow(skb);
+		rcu_read_unlock();
 		return NETDEV_TX_OK;
 	}
 #endif
@@ -48,7 +50,6 @@ netdev_tx_t br_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 	skb_reset_mac_header(skb);
 	skb_pull(skb, ETH_HLEN);
 
-	rcu_read_lock();
 	if (is_broadcast_ether_addr(dest))
 		br_flood_deliver(br, skb);
 	else if (is_multicast_ether_addr(dest)) {