tcp: refactor CA_Loss state processing

Consolidate all of TCP CA_Loss state processing in
tcp_fastretrans_alert() into a new function called tcp_process_loss().
This is to prepare the new F-RTO implementation in the next patch.

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