af_rxrpc: Add sysctls for configuring RxRPC parameters

Add sysctls for configuring RxRPC protocol handling, specifically controls on
delays before ack generation, the delay before resending a packet, the maximum
lifetime of a call and the expiration times of calls, connections and
transports that haven't been recently used.

More info added in Documentation/networking/rxrpc.txt.

Signed-off-by: David Howells <dhowells@redhat.com>
diff --git a/net/rxrpc/ar-skbuff.c b/net/rxrpc/ar-skbuff.c
index de755e0..af9f4fd 100644
--- a/net/rxrpc/ar-skbuff.c
+++ b/net/rxrpc/ar-skbuff.c
@@ -83,6 +83,11 @@
 		rxrpc_request_final_ACK(call);
 	} else if (atomic_dec_and_test(&call->ackr_not_idle) &&
 		   test_and_clear_bit(RXRPC_CALL_TX_SOFT_ACK, &call->flags)) {
+		/* We previously soft-ACK'd some received packets that have now
+		 * been consumed, so send a hard-ACK if no more packets are
+		 * immediately forthcoming to allow the transmitter to free up
+		 * its Tx bufferage.
+		 */
 		_debug("send Rx idle ACK");
 		__rxrpc_propose_ACK(call, RXRPC_ACK_IDLE, sp->hdr.serial,
 				    true);