Revert "ipv4: arp announce, arp_proxy and windows ip conflict verification"

This reverts commit 73ce7b01b4496a5fbf9caf63033c874be692333f.

After discovering that we don't listen to gratuitious arps in 2.6.30
I tracked the failure down to this commit.

The patch makes absolutely no sense.  RFC2131 RFC3927 and RFC5227.
are all in agreement that an arp request with sip == 0 should be used
for the probe (to prevent learning) and an arp request with sip == tip
should be used for the gratitous announcement that people can learn
from.

It appears the author of the broken patch got those two cases confused
and modified the code to drop all gratuitous arp traffic.  Ouch!

Cc: stable@kernel.org
Signed-off-by: Eric W. Biederman <ebiederm@aristanetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/arp.c

@@ -801,11 +801,8 @@ static int arp_process(struct sk_buff *skb)
  *  cache.
  */
 
-	/*
-	 *  Special case: IPv4 duplicate address detection packet (RFC2131)
-	 *  and Gratuitous ARP/ARP Announce. (RFC3927, Section 2.4)
-	 */
-	if (sip == 0 || tip == sip) {
+	/* Special case: IPv4 duplicate address detection packet (RFC2131) */
+	if (sip == 0) {
 		if (arp->ar_op == htons(ARPOP_REQUEST) &&
 		    inet_addr_type(net, tip) == RTN_LOCAL &&
 		    !arp_ignore(in_dev, sip, tip))