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/ipv4/udp.c b/net/ipv4/udp.c
index b3f7e8c..28cb2d7 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1413,7 +1413,7 @@
 		}
 	}
 
-	if (sk->sk_filter) {
+	if (rcu_dereference_raw(sk->sk_filter)) {
 		if (udp_lib_checksum_complete(skb))
 			goto drop;
 	}