tcp: mitigate a side channel for detection of TCP connections

[tuexen]

Summary

If a blind attacker wants to guess by sending ACK segments if there exists a TCP connection , this might trigger a challenge ACK on an existing TCP connection. To make this hit non-observable for the attacker, also increment the global counter, which would have been incremented if it would have been a non-hit.
This issue was reported as issue number 11 in Keyu Man et al.: SCAD: Towards a Universal and Automated Network Side-Channel Vulnerability Detection