net-tcp: retire TFO_SERVER_WO_SOCKOPT2 config

TFO_SERVER_WO_SOCKOPT2 was intended for debugging purposes during
Fast Open development. Remove this config option and also
update/clean-up the documentation of the Fast Open sysctl.

Reported-by: Piotr Jurkiewicz <piotr.jerzy.jurkiewicz@gmail.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index 55513e6..989a362 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -211,24 +211,19 @@
 	 * we can only allow the backlog to be adjusted.
 	 */
 	if (old_state != TCP_LISTEN) {
-		/* Check special setups for testing purpose to enable TFO w/o
-		 * requiring TCP_FASTOPEN sockopt.
+		/* Enable TFO w/o requiring TCP_FASTOPEN socket option.
 		 * Note that only TCP sockets (SOCK_STREAM) will reach here.
-		 * Also fastopenq may already been allocated because this
-		 * socket was in TCP_LISTEN state previously but was
-		 * shutdown() (rather than close()).
+		 * Also fastopen backlog may already been set via the option
+		 * because the socket was in TCP_LISTEN state previously but
+		 * was shutdown() rather than close().
 		 */
-		if ((sysctl_tcp_fastopen & TFO_SERVER_ENABLE) != 0 &&
+		if ((sysctl_tcp_fastopen & TFO_SERVER_WO_SOCKOPT1) &&
+		    (sysctl_tcp_fastopen & TFO_SERVER_ENABLE) &&
 		    !inet_csk(sk)->icsk_accept_queue.fastopenq.max_qlen) {
-			if ((sysctl_tcp_fastopen & TFO_SERVER_WO_SOCKOPT1) != 0)
-				fastopen_queue_tune(sk, backlog);
-			else if ((sysctl_tcp_fastopen &
-				  TFO_SERVER_WO_SOCKOPT2) != 0)
-				fastopen_queue_tune(sk,
-				    ((uint)sysctl_tcp_fastopen) >> 16);
-
+			fastopen_queue_tune(sk, backlog);
 			tcp_fastopen_init_key_once(true);
 		}
+
 		err = inet_csk_listen_start(sk, backlog);
 		if (err)
 			goto out;