tcp: Fix thinko making the not-shiftable to cover S|R as well

S|R won't result in S if just SACK is received. DSACK is
another story (but it is covered correctly already).

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