why fastopen failed from client perspective
TFO_STATUS_UNSPEC,
catch-all
TFO_COOKIE_UNAVAILABLE,
if not in TFO_CLIENT_NO_COOKIE mode
TFO_DATA_NOT_ACKED,
SYN-ACK did not ack SYN data
TFO_SYN_RETRANSMITTED,
SYN-ACK did not ack SYN data after timeout