tcp: socket option to set TCP fast open key

New socket option TCP_FASTOPEN_KEY to allow different keys per
listener.  The listener by default uses the global key until the
socket option is set.  The key is a 16 bytes long binary data. This
option has no effect on regular non-listener TCP sockets.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/request_sock.h b/include/net/request_sock.h
index 23e2205..3470155 100644
--- a/include/net/request_sock.h
+++ b/include/net/request_sock.h
@@ -150,6 +150,8 @@ struct fastopen_queue {
 	spinlock_t	lock;
 	int		qlen;		/* # of pending (TCP_SYN_RECV) reqs */
 	int		max_qlen;	/* != 0 iff TFO is currently enabled */
+
+	struct tcp_fastopen_context __rcu *ctx; /* cipher context for cookie */
 };
 
 /** struct request_sock_queue - queue of request_socks