mptcp: add TCP_FASTOPEN_NO_COOKIE support

The goal of this socket option is to configure MPTCP + TFO without
cookie per socket.

It was already possible to enable TFO without a cookie per netns by
setting net.ipv4.tcp_fastopen sysctl knob to the right value. Per route
was also supported by setting 'fastopen_no_cookie' option. This patch
adds a per socket support like it is possible to do with TCP thanks to
TCP_FASTOPEN_NO_COOKIE socket option.

The only thing to do here is to relay the request to the first subflow
like it is already done for TCP_FASTOPEN_CONNECT.
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

@@ -560,6 +560,7 @@ static bool mptcp_supported_sockopt(int level, int optname)
 		case TCP_TX_DELAY:
 		case TCP_INQ:
 		case TCP_FASTOPEN_CONNECT:
+		case TCP_FASTOPEN_NO_COOKIE:
 			return true;
 		}
 
@@ -568,8 +569,8 @@ static bool mptcp_supported_sockopt(int level, int optname)
 		/* TCP_REPAIR, TCP_REPAIR_QUEUE, TCP_QUEUE_SEQ, TCP_REPAIR_OPTIONS,
 		 * TCP_REPAIR_WINDOW are not supported, better avoid this mess
 		 */
-		/* TCP_FASTOPEN_KEY, TCP_FASTOPEN, TCP_FASTOPEN_NO_COOKIE,
-		 * are not supported fastopen is currently unsupported
+		/* TCP_FASTOPEN_KEY, TCP_FASTOPEN are not supported because
+		 * fastopen for the listener side is currently unsupported
 		 */
 	}
 	return false;
@@ -811,6 +812,7 @@ 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:
+	case TCP_FASTOPEN_NO_COOKIE:
 		return mptcp_setsockopt_first_sf_only(msk, SOL_TCP, optname,
 						      optval, optlen);
 	}
@@ -1175,6 +1177,7 @@ static int mptcp_getsockopt_sol_tcp(struct mptcp_sock *msk, int optname,
 	case TCP_CC_INFO:
 	case TCP_DEFER_ACCEPT:
 	case TCP_FASTOPEN_CONNECT:
+	case TCP_FASTOPEN_NO_COOKIE:
 		return mptcp_getsockopt_first_sf_only(msk, SOL_TCP, optname,
 						      optval, optlen);
 	case TCP_INQ: