net: Don't overwrite waiting packets with asynchronous replies

Peter originally sent a fix, but it breaks a number of other things.
This addresses the original reported issue in a different way.

That report was:

> U-Boot has 1 common buffer to send Ethernet frames, pointed to by
> net_tx_packet.  When sending to an IP address without knowing the MAC
> address, U-Boot makes an ARP request (using the arp_tx_packet buffer)
> to find out the MAC address of the IP addressr. When a matching ARP
> reply is received, U-Boot continues sending the frame stored in the
> net_tx_packet buffer.
>
> However, in the mean time, if U-Boot needs to send out any network
> packets (e.g. replying ping packets or ARP requests for its own IP
> address etc.), it will use the net_tx_packet buffer to prepare the
> new packet. Thus this buffer is no longer the original packet meant
> to be transmitted after the ARP reply. The original packet will be
> lost.

This instead uses the ARP tx buffer to send async replies in the case
where we are actively waiting for an ARP reply.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>

Reported-by: Tran Tien Dat <peter.trantiendat@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>

net/net.c

@@ -799,6 +799,14 @@ void net_set_timeout_handler(ulong iv, thand_f *f)
 	}
 }
 
+uchar *net_get_async_tx_pkt_buf(void)
+{
+	if (arp_is_waiting())
+		return arp_tx_packet; /* If we are waiting, we already sent */
+	else
+		return net_tx_packet;
+}
+
 int net_send_udp_packet(uchar *ether, struct in_addr dest, int dport, int sport,
 		int payload_len)
 {