mptcp: add MSG_FASTOPEN sendmsg flag support

Since commit 54f1944 ("mptcp: factor out mptcp_connect()"), all the
infrastructure is now in place to support the MSG_FASTOPEN flag, we
just need to call into the fastopen path in mptcp_sendmsg().

Co-developed-by: Benjamin Hesmans <benjamin.hesmans@tessares.net>
Signed-off-by: Benjamin Hesmans <benjamin.hesmans@tessares.net>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Dmytro Shytyi <dmytro@shytyi.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.c

@@ -1711,17 +1711,14 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	int ret = 0;
 	long timeo;
 
-	/* we don't support FASTOPEN yet */
-	if (msg->msg_flags & MSG_FASTOPEN)
-		return -EOPNOTSUPP;
-
 	/* silently ignore everything else */
-	msg->msg_flags &= MSG_MORE | MSG_DONTWAIT | MSG_NOSIGNAL;
+	msg->msg_flags &= MSG_MORE | MSG_DONTWAIT | MSG_NOSIGNAL | MSG_FASTOPEN;
 
 	lock_sock(sk);
 
 	ssock = __mptcp_nmpc_socket(msk);
-	if (unlikely(ssock && inet_sk(ssock->sk)->defer_connect)) {
+	if (unlikely(ssock && (inet_sk(ssock->sk)->defer_connect ||
+			       msg->msg_flags & MSG_FASTOPEN))) {
 		int copied_syn = 0;
 
 		ret = mptcp_sendmsg_fastopen(sk, ssock->sk, msg, len, &copied_syn);