net: forward_alloc_get depends on CONFIG_MPTCP

(struct proto)->sk_forward_alloc is currently only used by MPTCP.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -1206,7 +1206,9 @@ struct proto {
 	unsigned int		inuse_idx;
 #endif
 
+#if IS_ENABLED(CONFIG_MPTCP)
 	int			(*forward_alloc_get)(const struct sock *sk);
+#endif
 
 	bool			(*stream_memory_free)(const struct sock *sk, int wake);
 	bool			(*sock_is_readable)(struct sock *sk);
@@ -1295,10 +1297,11 @@ INDIRECT_CALLABLE_DECLARE(bool tcp_stream_memory_free(const struct sock *sk, int
 
 static inline int sk_forward_alloc_get(const struct sock *sk)
 {
-	if (!sk->sk_prot->forward_alloc_get)
-		return sk->sk_forward_alloc;
-
+#if IS_ENABLED(CONFIG_MPTCP)
+	if (sk->sk_prot->forward_alloc_get)
 		return sk->sk_prot->forward_alloc_get(sk);
+#endif
+	return sk->sk_forward_alloc;
 }
 
 static inline bool __sk_stream_memory_free(const struct sock *sk, int wake)