net: ipv6: Add IPv6 support to the ping socket.

This adds the ability to send ICMPv6 echo requests without a
raw socket. The equivalent ability for ICMPv4 was added in
2011.

Instead of having separate code paths for IPv4 and IPv6, make
most of the code in net/ipv4/ping.c dual-stack and only add a
few IPv6-specific bits (like the protocol definition) to a new
net/ipv6/ping.c. Hopefully this will reduce divergence and/or
duplication of bugs in the future.

Caveats:

- Setting options via ancillary data (e.g., using IPV6_PKTINFO
  to specify the outgoing interface) is not yet supported.
- There are no separate security settings for IPv4 and IPv6;
  everything is controlled by /proc/net/ipv4/ping_group_range.
- The proc interface does not yet display IPv6 ping sockets
  properly.

Tested with a patched copy of ping6 and using raw socket calls.
Compiles and works with all of CONFIG_IPV6={n,m,y}.

Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/icmp.c

@@ -939,7 +939,8 @@ error:
 void icmp_err(struct sk_buff *skb, u32 info)
 {
 	struct iphdr *iph = (struct iphdr *)skb->data;
-	struct icmphdr *icmph = (struct icmphdr *)(skb->data+(iph->ihl<<2));
+	int offset = iph->ihl<<2;
+	struct icmphdr *icmph = (struct icmphdr *)(skb->data + offset);
 	int type = icmp_hdr(skb)->type;
 	int code = icmp_hdr(skb)->code;
 	struct net *net = dev_net(skb->dev);
@@ -949,7 +950,7 @@ void icmp_err(struct sk_buff *skb, u32 info)
 	 * triggered by ICMP_ECHOREPLY which sent from kernel.
 	 */
 	if (icmph->type != ICMP_ECHOREPLY) {
-		ping_err(skb, info);
+		ping_err(skb, offset, info);
 		return;
 	}