message: Fix formula for credit computation
diff: @@ -106,7 +106,7 @@ void inbound_path::emit_ack_batch(local_actor* self, long queued_items,
  auto x = stats.calculate(cycle, complexity);
  // Hand out enough credit to fill our queue for 2 cycles.
  auto credit = std::max((x.max_throughput * 2)
  - - (assigned_credit - queued_items),
  + - (assigned_credit + queued_items),
  0l);
  auto batch_size = static_cast<int32_t>(x.items_per_batch);
  if (credit != 0)
  