https://bugs.winehq.org/show_bug.cgi?id=41753
--- Comment #13 from Bruno Jesus 00cpxxx@gmail.com --- adapterAddressesFromIndex already fills the prefix length for you. And this functions is called by GetAdaptersAddresses. So you just need to find out where it is stuffing the information.
http://source.winehq.org/source/dlls/iphlpapi/iphlpapi_main.c#1450