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/transp_v6.h b/include/net/transp_v6.h
index 48660e5..b927413d 100644
--- a/include/net/transp_v6.h
+++ b/include/net/transp_v6.h
@@ -32,8 +32,13 @@
 
 int udpv6_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len);
 
-int ip6_datagram_recv_ctl(struct sock *sk, struct msghdr *msg,
-			  struct sk_buff *skb);
+/* this does all the common and the specific ctl work */
+void 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 ip6_datagram_send_ctl(struct net *net, struct sock *sk, struct msghdr *msg,
 			  struct flowi6 *fl6, struct ipv6_txoptions *opt,