tcp: clean up use of jiffies in tcp_rcv_rtt_measure()

Clean up a reference to jiffies in tcp_rcv_rtt_measure() that should
instead reference tcp_time_stamp. Since the result of the subtraction
is passed into a function taking u32, this should not change any
behavior (and indeed the generated assembly does not change on
x86_64). However, it seems worth cleaning this up for consistency and
clarity (and perhaps to avoid bugs if this is copied and pasted
somewhere else).
Signed-off-by: NNeal Cardwell <ncardwell@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -495,7 +495,7 @@ static inline void tcp_rcv_rtt_measure(struct tcp_sock *tp)
 		goto new_measure;
 	if (before(tp->rcv_nxt, tp->rcv_rtt_est.seq))
 		return;
-	tcp_rcv_rtt_update(tp, jiffies - tp->rcv_rtt_est.time, 1);
+	tcp_rcv_rtt_update(tp, tcp_time_stamp - tp->rcv_rtt_est.time, 1);
 
 new_measure:
 	tp->rcv_rtt_est.seq = tp->rcv_nxt + tp->rcv_wnd;