tcp: consolidate tcp_mark_skb_lost and tcp_skb_mark_lost

tcp_skb_mark_lost is used by RFC6675-SACK and can easily be replaced
with the new tcp_mark_skb_lost handler.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>