tcp: Make SYN ACK RTO tunable by BPF programs with TFO

Instead of the hardcoded TCP_TIMEOUT_INIT, this diff calls tcp_timeout_init
to initiate req->timeout like the non TFO SYN ACK case.

Tested using the following packetdrill script, on a host with a BPF
program that sets the initial connect timeout to 10ms.

`../../common/defaults.sh`

// Initialize connection
    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
   +0 setsockopt(3, SOL_TCP, TCP_FASTOPEN, [1], 4) = 0
   +0 bind(3, ..., ...) = 0
   +0 listen(3, 1) = 0

   +0 < S 0:0(0) win 32792 <mss 1000,sackOK,FO TFO_COOKIE>
   +0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK>
   +.01 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK>
   +.02 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK>
   +.04 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK>
   +.01 < . 1:1(0) ack 1 win 32792

   +0 accept(3, ..., ...) = 4
Signed-off-by: NJie Meng <jmeng@fb.com>
Acked-by: NNeal Cardwell <ncardwell@google.com>
Acked-by: NYuchung Cheng <ycheng@google.com>
Acked-by: NMartin KaFai Lau <kafai@fb.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_fastopen.c

@@ -272,8 +272,9 @@ static struct sock *tcp_fastopen_create_child(struct sock *sk,
 	 * The request socket is not added to the ehash
 	 * because it's been added to the accept queue directly.
 	 */
+	req->timeout = tcp_timeout_init(child);
 	inet_csk_reset_xmit_timer(child, ICSK_TIME_RETRANS,
-				  TCP_TIMEOUT_INIT, TCP_RTO_MAX);
+				  req->timeout, TCP_RTO_MAX);
 
 	refcount_set(&req->rsk_refcnt, 2);
 

net/ipv4/tcp_timer.c

@@ -428,7 +428,7 @@ static void tcp_fastopen_synack_timer(struct sock *sk, struct request_sock *req)
 	if (!tp->retrans_stamp)
 		tp->retrans_stamp = tcp_time_stamp(tp);
 	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
-			  TCP_TIMEOUT_INIT << req->num_timeout, TCP_RTO_MAX);
+			  req->timeout << req->num_timeout, TCP_RTO_MAX);
 }