[TCP] FRTO: Incorrectly clears TCPCB_EVER_RETRANS bit

FRTO was slightly too brave... Should only clear
TCPCB_SACKED_RETRANS bit.

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