cs16/squid: correctly allow only the /24 subnet of the 192.168.101.0 network

the notation previously used would have allowed the whole /16, despite a
single address being specified.

This was also causing warnings:

2018/10/09 06:25:09| aclIpParseIpData: WARNING: Netmask masks away part of the specified IP in '192.168.101.16/16'

Signed-off-by: Mattia Rizzolo <mattia@debian.org>