[TCP]: congestion control API pass RTT in microseconds

This patch changes the API for the callback that is done after an ACK is
received. It solves a couple of issues:

  * Some congestion controls want higher resolution value of RTT
    (controlled by TCP_CONG_RTT_SAMPLE flag). These don't really want a ktime, but
    all compute a RTT in microseconds.

  * Other congestion control could use RTT at jiffies resolution.

To keep API consistent the units should be the same for both cases, just the
resolution should change.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/tcp_yeah.c b/net/ipv4/tcp_yeah.c
index c04b7c6..c107fba 100644
--- a/net/ipv4/tcp_yeah.c
+++ b/net/ipv4/tcp_yeah.c
@@ -58,7 +58,7 @@
 }
 
 
-static void tcp_yeah_pkts_acked(struct sock *sk, u32 pkts_acked, ktime_t last)
+static void tcp_yeah_pkts_acked(struct sock *sk, u32 pkts_acked, s32 rtt_us)
 {
 	const struct inet_connection_sock *icsk = inet_csk(sk);
 	struct yeah *yeah = inet_csk_ca(sk);
@@ -66,7 +66,7 @@
 	if (icsk->icsk_ca_state == TCP_CA_Open)
 		yeah->pkts_acked = pkts_acked;
 
-	tcp_vegas_pkts_acked(sk, pkts_acked, last);
+	tcp_vegas_pkts_acked(sk, pkts_acked, rtt_us);
 }
 
 static void tcp_yeah_cong_avoid(struct sock *sk, u32 ack,