tcp: convert cached rtt from usec to jiffies when feeding initial rto

Currently it's converted into msecs, thus HZ=1000 intact.
Signed-off-by: NKonstantin Khlebnikov <khlebnikov@yandex-team.ru>
Fixes: 740b0f18 ("tcp: switch rtt estimations to usec resolution")
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_metrics.c

@@ -550,7 +550,7 @@ void tcp_init_metrics(struct sock *sk)
 	 */
 	if (crtt > tp->srtt_us) {
 		/* Set RTO like tcp_rtt_estimator(), but from cached RTT. */
-		crtt /= 8 * USEC_PER_MSEC;
+		crtt /= 8 * USEC_PER_SEC / HZ;
 		inet_csk(sk)->icsk_rto = crtt + max(2 * crtt, tcp_rto_min(sk));
 	} else if (tp->srtt_us == 0) {
 		/* RFC6298: 5.7 We've failed to get a valid RTT sample from