net: add __rcu annotation to sk_filter

Add __rcu annotation to :
        (struct sock)->sk_filter

And use appropriate rcu primitives to reduce sparse warnings if
CONFIG_SPARSE_RCU_POINTER=y

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/raw.c b/net/ipv6/raw.c
index 45e6efb7..86c3952 100644
--- a/net/ipv6/raw.c
+++ b/net/ipv6/raw.c
@@ -373,7 +373,7 @@
 
 static inline int rawv6_rcv_skb(struct sock * sk, struct sk_buff * skb)
 {
-	if ((raw6_sk(sk)->checksum || sk->sk_filter) &&
+	if ((raw6_sk(sk)->checksum || rcu_dereference_raw(sk->sk_filter)) &&
 	    skb_checksum_complete(skb)) {
 		atomic_inc(&sk->sk_drops);
 		kfree_skb(skb);