ipv6: export a stub for IPv6 symbols used by vxlan

In case IPv6 is compiled as a module, introduce a stub
for ipv6_sock_mc_join and ipv6_sock_mc_drop etc.. It will be used
by vxlan module. Suggested by Ben.

This is an ugly but easy solution for now.

Cc: Ben Hutchings <bhutchings@solarflare.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/addrconf.h b/include/net/addrconf.h
index 43fa31a..5339cab 100644
--- a/include/net/addrconf.h
+++ b/include/net/addrconf.h
@@ -141,6 +141,21 @@
 			 const struct in6_addr *src_addr);
 
 void ipv6_mc_dad_complete(struct inet6_dev *idev);
+
+/* A stub used by vxlan module. This is ugly, ideally these
+ * symbols should be built into the core kernel.
+ */
+struct ipv6_stub {
+	int (*ipv6_sock_mc_join)(struct sock *sk, int ifindex,
+				 const struct in6_addr *addr);
+	int (*ipv6_sock_mc_drop)(struct sock *sk, int ifindex,
+				 const struct in6_addr *addr);
+	int (*ipv6_dst_lookup)(struct sock *sk, struct dst_entry **dst,
+				struct flowi6 *fl6);
+	void (*udpv6_encap_enable)(void);
+};
+extern const struct ipv6_stub *ipv6_stub __read_mostly;
+
 /*
  * identify MLD packets for MLD filter exceptions
  */