tcp: fix tcp_rearm_rto()

skbs in (re)transmit queue no longer have a copy of jiffies
at the time of the transmit : skb->skb_mstamp is now in usec unit,
with no correlation to tcp_jiffies32.

We have to convert rto from jiffies to usec, compute a time difference
in usec, then convert the delta to HZ units.

Fixes: 9a568de4818d ("tcp: switch TCP TS option (RFC 7323) to 1ms clock")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -3002,14 +3002,14 @@ void tcp_rearm_rto(struct sock *sk)
 		if (icsk->icsk_pending == ICSK_TIME_REO_TIMEOUT ||
 		    icsk->icsk_pending == ICSK_TIME_LOSS_PROBE) {
 			struct sk_buff *skb = tcp_write_queue_head(sk);
-			const u32 rto_time_stamp =
-				tcp_skb_timestamp(skb) + rto;
-			s32 delta = (s32)(rto_time_stamp - tcp_jiffies32);
-			/* delta may not be positive if the socket is locked
+			u64 rto_time_stamp = skb->skb_mstamp +
+					     jiffies_to_usecs(rto);
+			s64 delta_us = rto_time_stamp - tp->tcp_mstamp;
+			/* delta_us may not be positive if the socket is locked
 			 * when the retrans timer fires and is rescheduled.
 			 */
-			if (delta > 0)
-				rto = delta;
+			if (delta_us > 0)
+				rto = usecs_to_jiffies(delta_us);
 		}
 		inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, rto,
 					  TCP_RTO_MAX);