tcp: count sacked packets in tcp_sacktag_state

Add sack_delivered to tcp_sacktag_state and count the number of sacked
and dsacked packets. This is pure refactor for future patches to improve
tracking delivered counts.

Signed-off-by: Yousuk Seung <ysseung@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>