tcp: Fix MD5 signature checking on IPv4 mapped sockets

Fix MD5 signature checking so that an IPv4 active open
to an IPv6 socket can succeed.  In particular, use the
correct address family's signature generation function
for the SYN/ACK.

Reported-by:   Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: John Dykstra <john.dykstra1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1425,6 +1425,11 @@ struct tcp_request_sock_ops {
 #ifdef CONFIG_TCP_MD5SIG
 	struct tcp_md5sig_key	*(*md5_lookup) (struct sock *sk,
 						struct request_sock *req);
+	int			(*calc_md5_hash) (char *location,
+						  struct tcp_md5sig_key *md5,
+						  struct sock *sk,
+						  struct request_sock *req,
+						  struct sk_buff *skb);
 #endif
 };