tcp: move tcp_mark_skb_lost

A pure refactor to move tcp_mark_skb_lost to tcp_input.c to prepare
for the later loss marking consolidation.

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>