mptcp: preserve const qualifier in mptcp_sk()

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

We need to change few things to avoid build errors:

mptcp_set_datafin_timeout() and mptcp_rtx_head() have to accept
non-const sk pointers.

@msk local variable in mptcp_pending_tail() must be const.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Matthieu Baerts <matthieu.baerts@tessares.net>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -459,7 +459,7 @@ static bool mptcp_pending_data_fin(struct sock *sk, u64 *seq)
 	return false;
 }
 
-static void mptcp_set_datafin_timeout(const struct sock *sk)
+static void mptcp_set_datafin_timeout(struct sock *sk)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	u32 retransmits;

net/mptcp/protocol.h

@@ -334,10 +334,7 @@ static inline void msk_owned_by_me(const struct mptcp_sock *msk)
 	sock_owned_by_me((const struct sock *)msk);
 }
 
-static inline struct mptcp_sock *mptcp_sk(const struct sock *sk)
-{
-	return (struct mptcp_sock *)sk;
-}
+#define mptcp_sk(ptr) container_of_const(ptr, struct mptcp_sock, sk.icsk_inet.sk)
 
 /* the msk socket don't use the backlog, also account for the bulk
  * free memory
@@ -371,7 +368,7 @@ static inline struct mptcp_data_frag *mptcp_send_next(struct sock *sk)
 
 static inline struct mptcp_data_frag *mptcp_pending_tail(const struct sock *sk)
 {
-	struct mptcp_sock *msk = mptcp_sk(sk);
+	const struct mptcp_sock *msk = mptcp_sk(sk);
 
 	if (!msk->first_pending)
 		return NULL;
@@ -382,7 +379,7 @@ static inline struct mptcp_data_frag *mptcp_pending_tail(const struct sock *sk)
 	return list_last_entry(&msk->rtx_queue, struct mptcp_data_frag, list);
 }
 
-static inline struct mptcp_data_frag *mptcp_rtx_head(const struct sock *sk)
+static inline struct mptcp_data_frag *mptcp_rtx_head(struct sock *sk)
 {
 	struct mptcp_sock *msk = mptcp_sk(sk);