dccp: preserve const qualifier in dccp_sk()

We can change dccp_sk() to propagate its argument const qualifier,
thanks to container_of_const().
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/dccp.h

@@ -305,10 +305,8 @@ struct dccp_sock {
 	struct timer_list		dccps_xmit_timer;
 };
 
-static inline struct dccp_sock *dccp_sk(const struct sock *sk)
-{
-	return (struct dccp_sock *)sk;
-}
+#define dccp_sk(ptr)	container_of_const(ptr, struct dccp_sock, \
+					   dccps_inet_connection.icsk_inet.sk)
 
 static inline const char *dccp_role(const struct sock *sk)
 {