[TCP] FRTO: Prevent state inconsistency in corner cases

State could become inconsistent in two cases:

1) Userspace disabled FRTO by tuning sysctl when one of the TCP
   flows was in the middle of FRTO algorithm (and then RTO is
   again triggered)

2) SACK reneging occurs during FRTO algorithm

A simple solution is just to abort the previous FRTO when such
obscure condition occurs...

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>