updates these numbers in TTreePerfStats::FileReadEvent() (in preparation for multi TTree monitoring)

Currently TTreePerfStat reports the number of disk reads, by obtaining the total number of reads and their total size from the TFile, which will include calls from the TFile::Open() which are not strictly caused by TTree. This patch (also in preparation for multi TTree monitoring) updates these numbers in TTreePerfStats::FileReadEvent().