Added proper accounting of sacked_bytes and (per-ACK) delivered data.
Previous code was susceptible to improperly calculate these values, when
a) more than 3 (or 4, without TS) holes were to be reported
b) simplistic clients only sending a SACK block covering the last received data segment (simplistic IoT devices)
c) malicious receivers sending duplicate SACK blocks in the ACK to deflate pipe (possibly trigger burst transmissions by the sender).
As net.inet.tcp.rfc6675 controls this particular feature, and is disabled by default, the above issues are not critical.
This patch will allow more aspects of RFC6675 to be implemented. One is when Loss Recovery is invoked, the other is the Rescue Retransmission when TCP is application limited.