tcp: move ipv6_specific declaration to remove a warning

ipv6_specific should be declared in tcp include files,
not mptcp.

This removes the following warning :
  CHECK   net/ipv6/tcp_ipv6.c
net/ipv6/tcp_ipv6.c:78:42: warning: symbol 'ipv6_specific' was not declared. Should it be static?
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -933,6 +933,8 @@ static inline int tcp_v6_sdif(const struct sk_buff *skb)
 	return 0;
 }
 
+extern const struct inet_connection_sock_af_ops ipv6_specific;
+
 INDIRECT_CALLABLE_DECLARE(void tcp_v6_send_check(struct sock *sk, struct sk_buff *skb));
 INDIRECT_CALLABLE_DECLARE(int tcp_v6_rcv(struct sk_buff *skb));
 INDIRECT_CALLABLE_DECLARE(void tcp_v6_early_demux(struct sk_buff *skb));

net/mptcp/protocol.h

@@ -355,9 +355,6 @@ static inline void mptcp_subflow_tcp_fallback(struct sock *sk,
 }
 
 extern const struct inet_connection_sock_af_ops ipv4_specific;
-#if IS_ENABLED(CONFIG_MPTCP_IPV6)
-extern const struct inet_connection_sock_af_ops ipv6_specific;
-#endif
 
 void mptcp_proto_init(void);
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)