unnecessary rx callbacks when zero packets

[Konstantin Ananyev]

rx callbacks when zero packets
> 
> I noticed while looking at packet capture that currently the receive callbacks
> get called even if there are no packets. This seems unnecessary since if
> nb_rx is zero, then there are no packets to look at.  My one concern is that
> an application could be using callbacks as some form of scheduling mechanism
> which would be broken.

As I remember, original idea was to allow callbacks to inject new packets if needed.

> 
> The change would be:
> 
> 
> diff --git a/lib/ethdev/rte_ethdev.h b/lib/ethdev/rte_ethdev.h
> index 21e3a21903ec..f64bf977c46e 100644
> --- a/lib/ethdev/rte_ethdev.h
> +++ b/lib/ethdev/rte_ethdev.h
> @@ -6077,7 +6077,7 @@ rte_eth_rx_burst(uint16_t port_id, uint16_t queue_id,
>         nb_rx = p->rx_pkt_burst(qd, rx_pkts, nb_pkts);
> 
>  #ifdef RTE_ETHDEV_RXTX_CALLBACKS
> -       {
> +       if (nb_rx > 0) {
>                 void *cb;
> 
>                 /* rte_memory_order_release memory order was used when the