tcp: add server ip to encrypt cookie in fast open

Encrypt the cookie with both server and client IPv4 addresses,
such that multi-homed server will grant different cookies
based on both the source and destination IPs. No client change
is needed since cookie is opaque to the client.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 27b652f..09cb5c1 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1300,7 +1300,8 @@
 
 extern struct tcp_fastopen_context __rcu *tcp_fastopen_ctx;
 int tcp_fastopen_reset_cipher(void *key, unsigned int len);
-void tcp_fastopen_cookie_gen(__be32 addr, struct tcp_fastopen_cookie *foc);
+extern void tcp_fastopen_cookie_gen(__be32 src, __be32 dst,
+				    struct tcp_fastopen_cookie *foc);
 
 #define TCP_FASTOPEN_KEY_LENGTH 16