mptcp: Fix undefined mptcp_handle_ipv6_mapped for modular IPV6

If CONFIG_MPTCP=y, CONFIG_MPTCP_IPV6=n, and CONFIG_IPV6=m:

    ERROR: "mptcp_handle_ipv6_mapped" [net/ipv6/ipv6.ko] undefined!

This does not happen if CONFIG_MPTCP_IPV6=y, as CONFIG_MPTCP_IPV6
selects CONFIG_IPV6, and thus forces CONFIG_IPV6 builtin.

As exporting a symbol for an empty function would be a bit wasteful, fix
this by providing a dummy version of mptcp_handle_ipv6_mapped() for the
CONFIG_MPTCP_IPV6=n case.

Rename mptcp_handle_ipv6_mapped() to mptcpv6_handle_mapped(), to make it
clear this is a pure-IPV6 function, just like mptcpv6_init().

Fixes: cec37a6e41 ("mptcp: Handle MP_CAPABLE options for outgoing connections")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/mptcp.h

@@ -174,15 +174,12 @@ static inline bool mptcp_skb_can_collapse(const struct sk_buff *to,
 
 #endif /* CONFIG_MPTCP */
 
-void mptcp_handle_ipv6_mapped(struct sock *sk, bool mapped);
-
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
 int mptcpv6_init(void);
+void mptcpv6_handle_mapped(struct sock *sk, bool mapped);
 #elif IS_ENABLED(CONFIG_IPV6)
-static inline int mptcpv6_init(void)
-{
-	return 0;
-}
+static inline int mptcpv6_init(void) { return 0; }
+static inline void mptcpv6_handle_mapped(struct sock *sk, bool mapped) { }
 #endif
 
 #endif /* __NET_MPTCP_H */

net/ipv6/tcp_ipv6.c

@@ -239,7 +239,7 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr *uaddr,
 
 		icsk->icsk_af_ops = &ipv6_mapped;
 		if (sk_is_mptcp(sk))
-			mptcp_handle_ipv6_mapped(sk, true);
+			mptcpv6_handle_mapped(sk, true);
 		sk->sk_backlog_rcv = tcp_v4_do_rcv;
 #ifdef CONFIG_TCP_MD5SIG
 		tp->af_specific = &tcp_sock_ipv6_mapped_specific;
@@ -251,7 +251,7 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr *uaddr,
 			icsk->icsk_ext_hdr_len = exthdrlen;
 			icsk->icsk_af_ops = &ipv6_specific;
 			if (sk_is_mptcp(sk))
-				mptcp_handle_ipv6_mapped(sk, false);
+				mptcpv6_handle_mapped(sk, false);
 			sk->sk_backlog_rcv = tcp_v6_do_rcv;
 #ifdef CONFIG_TCP_MD5SIG
 			tp->af_specific = &tcp_sock_ipv6_specific;
@@ -1208,7 +1208,7 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 
 		inet_csk(newsk)->icsk_af_ops = &ipv6_mapped;
 		if (sk_is_mptcp(newsk))
-			mptcp_handle_ipv6_mapped(newsk, true);
+			mptcpv6_handle_mapped(newsk, true);
 		newsk->sk_backlog_rcv = tcp_v4_do_rcv;
 #ifdef CONFIG_TCP_MD5SIG
 		newtp->af_specific = &tcp_sock_ipv6_mapped_specific;

net/mptcp/subflow.c

@@ -585,9 +585,9 @@ subflow_default_af_ops(struct sock *sk)
 	return &subflow_specific;
 }
 
-void mptcp_handle_ipv6_mapped(struct sock *sk, bool mapped)
-{
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
+void mptcpv6_handle_mapped(struct sock *sk, bool mapped)
+{
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct inet_connection_sock_af_ops *target;
@@ -602,8 +602,8 @@ void mptcp_handle_ipv6_mapped(struct sock *sk, bool mapped)
 
 	subflow->icsk_af_ops = icsk->icsk_af_ops;
 	icsk->icsk_af_ops = target;
-#endif
 }
+#endif
 
 int mptcp_subflow_create_socket(struct sock *sk, struct socket **new_sock)
 {