rxrpc: Reduce the number of PING ACKs sent

We don't want to send a PING ACK for every new incoming call as that just
adds to the network traffic.  Instead, we send a PING ACK to the first
three that we receive and then once per second thereafter.

This could probably be made adjustable in future.
Signed-off-by: NDavid Howells <dhowells@redhat.com>

net/rxrpc/input.c

@@ -44,7 +44,10 @@ static void rxrpc_send_ping(struct rxrpc_call *call, struct sk_buff *skb,
 			    int skew)
 {
 	struct rxrpc_skb_priv *sp = rxrpc_skb(skb);
+	ktime_t now = skb->tstamp;
 
+	if (call->peer->rtt_usage < 3 ||
+	    ktime_before(ktime_add_ms(call->peer->rtt_last_req, 1000), now))
 		rxrpc_propose_ACK(call, RXRPC_ACK_PING, skew, sp->hdr.serial,
 				  true, true);
 }