tcp: use tp->total_rto to track number of linear timeouts in SYN_SENT state

In commit ccce324d ("tcp: make the first N SYN RTO backoffs linear")
David used icsk->icsk_backoff field to track the number of linear timeouts.

Since then, tp->total_rto has been added.

This commit uses tp->total_rto instead of icsk->icsk_backoff
so that tcp_ld_RTO_revert() no longer can trigger an overflow
in inet_csk_rto_backoff(). Other than the potential UBSAN
report, there was no issue because receiving an ICMP message
currently aborts the connect().

In the following patch, we want to adhere to RFC 6069
and RFC 1122 4.2.3.9.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: David Morley <morleyd@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_timer.c

@@ -626,7 +626,6 @@ void tcp_retransmit_timer(struct sock *sk)
 	 * implemented ftp to mars will work nicely. We will have to fix
 	 * the 120 second clamps though!
 	 */
-	icsk->icsk_backoff++;
 
 out_reset_timer:
 	/* If stream is thin, use linear timeouts. Since 'icsk_backoff' is
@@ -647,11 +646,12 @@ void tcp_retransmit_timer(struct sock *sk)
 				       tcp_rto_min(sk),
 				       TCP_RTO_MAX);
 	} else if (sk->sk_state != TCP_SYN_SENT ||
-		   icsk->icsk_backoff >
+		   tp->total_rto >
 		   READ_ONCE(net->ipv4.sysctl_tcp_syn_linear_timeouts)) {
 		/* Use normal (exponential) backoff unless linear timeouts are
 		 * activated.
 		 */
+		icsk->icsk_backoff++;
 		icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
 	}
 	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,