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>
diff --git a/net/arp.c b/net/arp.c
index ea685d9..b49c3d3 100644
--- a/net/arp.c
+++ b/net/arp.c
@@ -34,8 +34,7 @@
 int		arp_wait_tx_packet_size;
 ulong		arp_wait_timer_start;
 int		arp_wait_try;
-
-static uchar   *arp_tx_packet;	/* THE ARP transmit packet */
+uchar	       *arp_tx_packet; /* THE ARP transmit packet */
 static uchar	arp_tx_packet_buf[PKTSIZE_ALIGN + PKTALIGN];
 
 void arp_init(void)
@@ -126,6 +125,7 @@
 	struct arp_hdr *arp;
 	struct in_addr reply_ip_addr;
 	int eth_hdr_size;
+	uchar *tx_packet;
 
 	/*
 	 * We have to deal with two types of ARP packets:
@@ -182,8 +182,9 @@
 		    (net_read_ip(&arp->ar_spa).s_addr & net_netmask.s_addr))
 			udelay(5000);
 #endif
-		memcpy(net_tx_packet, et, eth_hdr_size + ARP_HDR_SIZE);
-		net_send_packet(net_tx_packet, eth_hdr_size + ARP_HDR_SIZE);
+		tx_packet = net_get_async_tx_pkt_buf();
+		memcpy(tx_packet, et, eth_hdr_size + ARP_HDR_SIZE);
+		net_send_packet(tx_packet, eth_hdr_size + ARP_HDR_SIZE);
 		return;
 
 	case ARPOP_REPLY:		/* arp reply */