[TCP]: No fackets_out/highest_sack tuning when SACK isn't enabled

This was found due to bug report from Cedric Le Goater though
it turned this turned out to be unrelated bug.

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