rxrpc: Propose, but don't immediately transmit, the final ACK for a call

The final ACK that closes out an rxrpc call needs to be transmitted by the
client unless we're going to follow up with a DATA packet for a new call on
the same channel (which implicitly ACK's the previous call, thereby saving
an ACK).

Currently, we don't do that, so if no follow on call is immediately
forthcoming, the server will resend the last DATA packet - at which point
rxrpc_conn_retransmit_call() will be triggered and will (re)send the final
ACK.  But the server has to hold on to the last packet until the ACK is
received, thereby holding up its resources.

Fix the client side to propose a delayed final ACK, to be transmitted after
a short delay, assuming the call isn't superseded by a new one.

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

net/rxrpc/recvmsg.c

@@ -144,13 +144,11 @@ static void rxrpc_end_rx_phase(struct rxrpc_call *call, rxrpc_serial_t serial)
 	trace_rxrpc_receive(call, rxrpc_receive_end, 0, call->rx_top);
 	ASSERTCMP(call->rx_hard_ack, ==, call->rx_top);
 
-#if 0 // TODO: May want to transmit final ACK under some circumstances anyway
 	if (call->state == RXRPC_CALL_CLIENT_RECV_REPLY) {
-		rxrpc_propose_ACK(call, RXRPC_ACK_IDLE, 0, serial, true, false,
+		rxrpc_propose_ACK(call, RXRPC_ACK_IDLE, 0, serial, false, true,
 				  rxrpc_propose_ack_terminal_ack);
-		rxrpc_send_ack_packet(call, false, NULL);
+		//rxrpc_send_ack_packet(call, false, NULL);
 	}
-#endif
 
 	write_lock_bh(&call->state_lock);