[XrdOss] Fix CodeQL warning about int overflow in multiplication

The multiplication is numq * xfrovhd, which if carried with ints
can overflow before being converted to unsigned long long. Convert
one of the operands to unsigned long long to avoid this.

src/XrdOss/XrdOssStage.cc

@@ -393,8 +393,9 @@ int XrdOssSys::CalcTime()
 
 int XrdOssSys::CalcTime(XrdOssStage_Req *req) // StageMutex lock held!
 {
+    unsigned long long numq = 1;
     unsigned long long tbytes = req->size + stgbytes/2;
-    int xfrtime, numq = 1;
+    int xfrtime;
     time_t now;
     XrdOssStage_Req *rqp = req;