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/ar-internal.h

@@ -142,10 +142,7 @@ struct rxrpc_host_header {
  */
 struct rxrpc_skb_priv {
 	union {
-		unsigned long	resend_at;	/* time in jiffies at which to resend */
-		struct {
-			u8	nr_jumbo;	/* Number of jumbo subpackets */
-		};
+		u8		nr_jumbo;	/* Number of jumbo subpackets */
 	};
 	union {
 		unsigned int	offset;		/* offset into buffer of next read */
@@ -663,6 +660,7 @@ extern const char rxrpc_recvmsg_traces[rxrpc_recvmsg__nr_trace][5];
 
 enum rxrpc_rtt_tx_trace {
 	rxrpc_rtt_tx_ping,
+	rxrpc_rtt_tx_data,
 	rxrpc_rtt_tx__nr_trace
 };
 
@@ -670,6 +668,7 @@ extern const char rxrpc_rtt_tx_traces[rxrpc_rtt_tx__nr_trace][5];
 
 enum rxrpc_rtt_rx_trace {
 	rxrpc_rtt_rx_ping_response,
+	rxrpc_rtt_rx_requested_ack,
 	rxrpc_rtt_rx__nr_trace
 };