Fixed arguments passed in the incorrect order in CFileSystemPassThru::AllocOptimalReadBuffer

Arguments are passed in the incorrect order in the function CFileSystemPassThru::AllocOptimalReadBuffer (nSize and nOffset get swapped), which can result in incorrect behavior. This interface is used in VMPI according to the comments in the header.

Anyone using the CFileSystemPassThru interface to override a filesystem function in the Client or Server DLL will likely run into this problem.