sctp: preserve const qualifier in sctp_sk()

We can change sctp_sk() to propagate its argument const qualifier,
thanks to container_of_const().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Cc: Xin Long <lucien.xin@gmail.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sctp/structs.h

@@ -242,10 +242,7 @@ struct sctp_sock {
 	int do_auto_asconf;
 };
 
-static inline struct sctp_sock *sctp_sk(const struct sock *sk)
-{
-       return (struct sctp_sock *)sk;
-}
+#define sctp_sk(ptr) container_of_const(ptr, struct sctp_sock, inet.sk)
 
 static inline struct sock *sctp_opt2sk(const struct sctp_sock *sp)
 {