[TCP]: Dropped unnecessary skb/sacked accessing in reneging

SACK reneging can be precalculated to a FLAG in clean_rtx_queue
which has the right skb looked up. This will help a bit in
future because skb->sacked access will be changed eventually,
changing it already won't hurt any.

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