[NETLINK]: use container_of instead

This could make future redesign of struct netlink_sock easier.
Signed-off-by: Denis Cheng <crquan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -88,7 +88,7 @@ struct netlink_sock {
 
 static inline struct netlink_sock *nlk_sk(struct sock *sk)
 {
-	return (struct netlink_sock *)sk;
+	return container_of(sk, struct netlink_sock, sk);
 }
 
 struct nl_pid_hash {