ipv6: make IPV6_RECVPKTINFO work for ipv4 datagrams

We currently don't report IPV6_RECVPKTINFO in cmsg access ancillary data
for IPv4 datagrams on IPv6 sockets.

This patch splits the ip6_datagram_recv_ctl into two functions, one
which handles both protocol families, AF_INET and AF_INET6, while the
ip6_datagram_recv_specific_ctl only handles IPv6 cmsg data.

ip6_datagram_recv_*_ctl never reported back any errors, so we can make
them return void. Also provide a helper for protocols which don't offer dual
personality to further use ip6_datagram_recv_ctl, which is exported to
modules.

I needed to shuffle the code for ping around a bit to make it easier to
implement dual personality for ping ipv6 sockets in future.

Reported-by: Gert Doering <gert@space.net>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/ping.h b/include/net/ping.h
index 7601365..026479b 100644
--- a/include/net/ping.h
+++ b/include/net/ping.h
@@ -33,8 +33,12 @@
 struct pingv6_ops {
 	int (*ipv6_recv_error)(struct sock *sk, struct msghdr *msg, int len,
 			       int *addr_len);
-	int (*ip6_datagram_recv_ctl)(struct sock *sk, struct msghdr *msg,
-				     struct sk_buff *skb);
+	void (*ip6_datagram_recv_common_ctl)(struct sock *sk,
+					     struct msghdr *msg,
+					     struct sk_buff *skb);
+	void (*ip6_datagram_recv_specific_ctl)(struct sock *sk,
+					       struct msghdr *msg,
+					       struct sk_buff *skb);
 	int (*icmpv6_err_convert)(u8 type, u8 code, int *err);
 	void (*ipv6_icmp_error)(struct sock *sk, struct sk_buff *skb, int err,
 				__be16 port, u32 info, u8 *payload);