[SOCK]: Adds a rcu_dereference() in sk_filter

It seems commit fda9ef5d679b07c9d9097aaf6ef7f069d794a8f9 introduced a RCU 
protection for sk_filter(), without a rcu_dereference()

Either we need a rcu_dereference(), either a comment should explain why we 
dont need it. I vote for the former.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -944,7 +944,7 @@ static inline int sk_filter(struct sock *sk, struct sk_buff *skb)
 		return err;
 	
 	rcu_read_lock_bh();
-	filter = sk->sk_filter;
+	filter = rcu_dereference(sk->sk_filter);
 	if (filter) {
 		unsigned int pkt_len = sk_run_filter(skb, filter->insns,
 				filter->len);