dnsmasq: Invoke new ipcalc with CIDR notation

The new rewritten ipcalc.sh understands 3 notations:

ipaddr/prefix ...
ipaddr/dotted-netmask ...
ipaddr dotted-netmask ...

meaning that the previous 4th non-standard notation of "ipaddr prefix"
will be dropped, alas that's the notation that dnsmasq currently uses.

This change has us using the first notation which is the most common.

This behavior came in as
https://github.com/openwrt/openwrt/commit/eda27e8382f3e5253defab5419e95d4b51f72de3
a long time ago.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>