rxrpc: Send an ACK after every few DATA packets we receive

Send an ACK if we haven't sent one for the last two packets we've received.
This keeps the other end apprised of where we've got to - which is
important if they're doing slow-start.

We do this in recvmsg so that we can dispatch a packet directly without the
need to wake up the background thread.

This should possibly be made configurable in future.

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

net/rxrpc/ar-internal.h

@@ -533,6 +533,8 @@ struct rxrpc_call {
 	u16			ackr_skew;	/* skew on packet being ACK'd */
 	rxrpc_serial_t		ackr_serial;	/* serial of packet being ACK'd */
 	rxrpc_seq_t		ackr_prev_seq;	/* previous sequence number received */
+	rxrpc_seq_t		ackr_consumed;	/* Highest packet shown consumed */
+	rxrpc_seq_t		ackr_seen;	/* Highest packet shown seen */
 	rxrpc_serial_t		ackr_ping;	/* Last ping sent */
 	ktime_t			ackr_ping_time;	/* Time last ping sent */
 
@@ -695,6 +697,7 @@ enum rxrpc_propose_ack_trace {
 	rxrpc_propose_ack_respond_to_ack,
 	rxrpc_propose_ack_respond_to_ping,
 	rxrpc_propose_ack_retry_tx,
+	rxrpc_propose_ack_rotate_rx,
 	rxrpc_propose_ack_terminal_ack,
 	rxrpc_propose_ack__nr_trace
 };