Fix for XrdOssReloc int overflow bug

I've recently hit this bug while converting old xrootd spaces to new format. It's been described in Savannah (https://savannah.cern.ch/bugs/?96364) quite a while ago but is still not fixed in xrootd 3.3.6
Basically, any file greater than 2G cannot be converted, moreover conversion stops when such a file is encountered and the only way to continue is to manually move the file out of the space.