rxrpc: Obtain RTT data by requesting ACKs on DATA packets

In addition to sending a PING ACK to gain RTT data, we can set the
RXRPC_REQUEST_ACK flag on a DATA packet and get a REQUESTED-ACK ACK.  The
ACK packet contains the serial number of the packet it is in response to,
so we can look through the Tx buffer for a matching DATA packet.

This requires that the data packets be stamped with the time of
transmission as a ktime rather than having the resend_at time in jiffies.

This further requires the resend code to do the resend determination in
ktimes and convert to jiffies to set the timer.

Signed-off-by: David Howells <dhowells@redhat.com>

net/rxrpc/sysctl.c

@@ -59,7 +59,7 @@ static struct ctl_table rxrpc_sysctl_table[] = {
 		.data		= &rxrpc_resend_timeout,
 		.maxlen		= sizeof(unsigned int),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec_ms_jiffies,
+		.proc_handler	= proc_dointvec,
 		.extra1		= (void *)&one,
 	},
 	{