rxrpc: Use before_eq() and friends to compare serial numbers

before_eq() and friends should be used to compare serial numbers (when not
checking for (non)equality) rather than casting to int, subtracting and
checking the result.

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

net/rxrpc/input.c

@@ -578,7 +578,7 @@ static void rxrpc_input_ack(struct rxrpc_call *call, struct sk_buff *skb,
 	}
 
 	/* Discard any out-of-order or duplicate ACKs. */
-	if ((int)sp->hdr.serial - (int)call->acks_latest <= 0) {
+	if (before_eq(sp->hdr.serial, call->acks_latest)) {
 		_debug("discard ACK %d <= %d",
 		       sp->hdr.serial, call->acks_latest);
 		return;