As stale data from a prior SACK loss recovery episode persists,
a subsequent reordering event may trigger a spurious rescue
retransmission, which in turn misbehaves due to other stale
data.
To prevent sending an erraneous rescue retransmission, clear
the sackhint.delivered_data value, once finished with a SACK
loss recovery.
Furthermore, initialize snd_fack to th_ack on a partial ack,
to pull it forward from whereever the last proper SACK may
have been.