panic in TCP Limited Transmit after RTO

[Noritoshi Demizu]

> Please try this patch.

I am sorry I have not reported the result.
I am trying to reproduce the scenario where snd_nxt is advanced
by more than t_maxseg in tcp_output() because of SACK blocks
when tcp_input() is doing Limited Transmit.  I tried tens of
times but I have not succeeded it yet.  I will report the result
later.

Regards,
Noritoshi Demizu