11.2 c says:

If the twoStepFlag bit of the flagField of the of the Sync message is TRUE,
indicating that a Follow_Up message will be received, then

        <offsetFromMaster> = <syncEventIngressTimestamp> -
       <preciseOriginTimestamp> - <meanPathDelay> -
       correctionField of Sync message -
       correctionField of Follow_Up message.

Before this patch, only the correctionField of the Sync message was
subtracted. We fix things by adding the Follow_Up correctionField to the
Sync correctionField.

Now m_to_s_dly as calculated in pp_servo_got_sync is:

t2 - t1 - sync_cf - follow_up_cf