af_rxrpc: Expose more RxRPC parameters via sysctls

Expose RxRPC parameters via sysctls to control the Rx window size, the Rx MTU
maximum size and the number of packets that can be glued into a jumbo packet.

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

Signed-off-by: David Howells <dhowells@redhat.com>
diff --git a/net/rxrpc/ar-call.c b/net/rxrpc/ar-call.c
index 1e0903a..6e4d58c 100644
--- a/net/rxrpc/ar-call.c
+++ b/net/rxrpc/ar-call.c
@@ -99,7 +99,7 @@
 	call->rx_data_expect = 1;
 	call->rx_data_eaten = 0;
 	call->rx_first_oos = 0;
-	call->ackr_win_top = call->rx_data_eaten + 1 + RXRPC_MAXACKS;
+	call->ackr_win_top = call->rx_data_eaten + 1 + rxrpc_rx_window_size;
 	call->creation_jif = jiffies;
 	return call;
 }