Thanks to Timo Voelker for finding and reporting the bug.

When using TFO and the SYN carries user data, the sending of
the SYN-ACK is delayed and accept returns. This allows the
application to provide user data to be put in the SYN-ACK.

Use the IANA assigned option number for the TCP Fast Open option.
Use EXP-FO instead of TO for using the deprecate experimental option.