tcp: Remove TCPCT

TCPCT uses option-number 253, reserved for experimental use and should
not be used in production environments.
Further, TCPCT does not fully implement RFC 6013.

As a nice side-effect, removing TCPCT increases TCP's performance for
very short flows:

Doing an apache-benchmark with -c 100 -n 100000, sending HTTP-requests
for files of 1KB size.

before this patch:
	average (among 7 runs) of 20845.5 Requests/Second
after:
	average (among 7 runs) of 21403.6 Requests/Second

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/tcp.h

@@ -90,9 +90,6 @@ struct tcp_options_received {
 		sack_ok : 4,	/* SACK seen on SYN packet		*/
 		snd_wscale : 4,	/* Window scaling received from sender	*/
 		rcv_wscale : 4;	/* Window scaling to send to receiver	*/
-	u8	cookie_plus:6,	/* bytes in authenticator/cookie option	*/
-		cookie_out_never:1,
-		cookie_in_always:1;
 	u8	num_sacks;	/* Number of SACK blocks		*/
 	u16	user_mss;	/* mss requested by user in ioctl	*/
 	u16	mss_clamp;	/* Maximal mss, negotiated at connection setup */
@@ -102,7 +99,6 @@ static inline void tcp_clear_options(struct tcp_options_received *rx_opt)
 {
 	rx_opt->tstamp_ok = rx_opt->sack_ok = 0;
 	rx_opt->wscale_ok = rx_opt->snd_wscale = 0;
-	rx_opt->cookie_plus = 0;
 }
 
 /* This is the max number of SACKS that we'll generate and process. It's safe
@@ -320,12 +316,6 @@ struct tcp_sock {
 	struct tcp_md5sig_info	__rcu *md5sig_info;
 #endif
 
-	/* When the cookie options are generated and exchanged, then this
-	 * object holds a reference to them (cookie_values->kref).  Also
-	 * contains related tcp_cookie_transactions fields.
-	 */
-	struct tcp_cookie_values  *cookie_values;
-
 /* TCP fastopen related information */
 	struct tcp_fastopen_request *fastopen_req;
 	/* fastopen_rsk points to request_sock that resulted in this big