[IPV4] ip_gre: sendto/recvfrom NBMA address

When GRE tunnel is in NBMA mode, this patch allows an application to use
a PF_PACKET socket to:
- send a packet to specific NBMA address with sendto()
- use recvfrom() to receive packet and check which NBMA address it came from

This is required to implement properly NHRP over GRE tunnel.
Signed-off-by: NTimo Teras <timo.teras@iki.fi>
Acked-by: NAlexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -1033,7 +1033,6 @@ static int ipgre_tunnel_change_mtu(struct net_device *dev, int new_mtu)
 	return 0;
 }
 
-#ifdef CONFIG_NET_IPGRE_BROADCAST
 /* Nice toy. Unfortunately, useless in real life :-)
    It allows to construct virtual multiprotocol broadcast "LAN"
    over the Internet, provided multicast routing is tuned.
@@ -1092,10 +1091,19 @@ static int ipgre_header(struct sk_buff *skb, struct net_device *dev,
 	return -t->hlen;
 }
 
+static int ipgre_header_parse(const struct sk_buff *skb, unsigned char *haddr)
+{
+	struct iphdr *iph = (struct iphdr*) skb_mac_header(skb);
+	memcpy(haddr, &iph->saddr, 4);
+	return 4;
+}
+
 static const struct header_ops ipgre_header_ops = {
 	.create	= ipgre_header,
+	.parse	= ipgre_header_parse,
 };
 
+#ifdef CONFIG_NET_IPGRE_BROADCAST
 static int ipgre_open(struct net_device *dev)
 {
 	struct ip_tunnel *t = netdev_priv(dev);
@@ -1197,6 +1205,8 @@ static int ipgre_tunnel_init(struct net_device *dev)
 			dev->stop = ipgre_close;
 		}
 #endif
+	} else {
+		dev->header_ops = &ipgre_header_ops;
 	}
 
 	if (!tdev && tunnel->parms.link)