mptcp: sockopt: make 'tcp_fastopen_connect' generic

There are other socket options that need to act only on the first
subflow, e.g. all TCP_FASTOPEN* socket options.

This is similar to the getsockopt version.

In the next commit, this new mptcp_setsockopt_first_sf_only() helper is
used by other another option.
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/mptcp/sockopt.c

@@ -769,17 +769,17 @@ static int mptcp_setsockopt_sol_tcp_defer(struct mptcp_sock *msk, sockptr_t optv
 	return tcp_setsockopt(listener->sk, SOL_TCP, TCP_DEFER_ACCEPT, optval, optlen);
 }
 
-static int mptcp_setsockopt_sol_tcp_fastopen_connect(struct mptcp_sock *msk, sockptr_t optval,
-						     unsigned int optlen)
+static int mptcp_setsockopt_first_sf_only(struct mptcp_sock *msk, int level, int optname,
+					  sockptr_t optval, unsigned int optlen)
 {
 	struct socket *sock;
 
-	/* Limit to first subflow */
+	/* Limit to first subflow, before the connection establishment */
 	sock = __mptcp_nmpc_socket(msk);
 	if (!sock)
 		return -EINVAL;
 
-	return tcp_setsockopt(sock->sk, SOL_TCP, TCP_FASTOPEN_CONNECT, optval, optlen);
+	return tcp_setsockopt(sock->sk, level, optname, optval, optlen);
 }
 
 static int mptcp_setsockopt_sol_tcp(struct mptcp_sock *msk, int optname,
@@ -811,7 +811,8 @@ static int mptcp_setsockopt_sol_tcp(struct mptcp_sock *msk, int optname,
 	case TCP_DEFER_ACCEPT:
 		return mptcp_setsockopt_sol_tcp_defer(msk, optval, optlen);
 	case TCP_FASTOPEN_CONNECT:
-		return mptcp_setsockopt_sol_tcp_fastopen_connect(msk, optval, optlen);
+		return mptcp_setsockopt_first_sf_only(msk, SOL_TCP, optname,
+						      optval, optlen);
 	}
 
 	return -EOPNOTSUPP;