tcp: fix SNMP TCP timeout under-estimation

Previously the SNMP TCPTIMEOUTS counter has inconsistent accounting:
1. It counts all SYN and SYN-ACK timeouts
2. It counts timeouts in other states except recurring timeouts and
   timeouts after fast recovery or disorder state.

Such selective accounting makes analysis difficult and complicated. For
example the monitoring system needs to collect many other SNMP counters
to infer the total amount of timeout events. This patch makes TCPTIMEOUTS
counter simply counts all the retransmit timeout (SYN or data or FIN).

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>

net/ipv4/tcp_timer.c

@@ -484,11 +484,12 @@ void tcp_retransmit_timer(struct sock *sk)
 		goto out_reset_timer;
 	}
 
+	__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEOUTS);
 	if (tcp_write_timeout(sk))
 		goto out;
 
 	if (icsk->icsk_retransmits == 0) {
-		int mib_idx;
+		int mib_idx = 0;
 
 		if (icsk->icsk_ca_state == TCP_CA_Recovery) {
 			if (tcp_is_sack(tp))
@@ -503,10 +504,9 @@ void tcp_retransmit_timer(struct sock *sk)
 				mib_idx = LINUX_MIB_TCPSACKFAILURES;
 			else
 				mib_idx = LINUX_MIB_TCPRENOFAILURES;
-		} else {
-			mib_idx = LINUX_MIB_TCPTIMEOUTS;
 		}
-		__NET_INC_STATS(sock_net(sk), mib_idx);
+		if (mib_idx)
+			__NET_INC_STATS(sock_net(sk), mib_idx);
 	}
 
 	tcp_enter_loss(sk);