Add a new sack hint to track the most recent and highest sacked sequence number.

This will be used by the incoming Enhanced RTT Khelp module.

Sponsored by:	FreeBSD Foundation
Submitted by:	David Hayes <dahayes at swin edu au>
Reviewed by:	bz and others (as part of a larger patch)
MFC after:	3 months

sys/netinet/tcp_sack.c

@@ -425,6 +425,7 @@ tcp_sack_doack(struct tcpcb *tp, struct tcpopt *to, tcp_seq th_ack)
 	 * are received.
 	 */
 	sblkp = &sack_blocks[num_sack_blks - 1];	/* Last SACK block */
+	tp->sackhint.last_sack_ack = sblkp->end;
 	if (SEQ_LT(tp->snd_fack, sblkp->start)) {
 		/*
 		 * The highest SACK block is beyond fack.  Append new SACK

sys/netinet/tcp_var.h

@@ -70,6 +70,7 @@ struct sackhole {
 struct sackhint {
 	struct sackhole	*nexthole;
 	int		sack_bytes_rexmit;
+	tcp_seq		last_sack_ack;	/* Most recent/largest sacked ack */
 
 	int		ispare;		/* explicit pad for 64bit alignment */
 	uint64_t	_pad[2];	/* 1 sacked_bytes, 1 TBD */