IN  PIRP_CONTEXT IrpContext,
  IN  PFCB Fcb,
  IN  LONGLONG FileOffset,
  OUT PULONG ByteCount

Routine Description:

    This routine looks through the mapping information for the file
    to find the logical diskoffset and number of bytes at that offset.
    We only deal with logical 2048 byte sectors here.

    If the mapping isn't present we will look it up on disk now.
    This routine assumes we are looking up a valid range in the file.  This
    routine raises if it can't find mapping for the file offset.

    The Fcb may not be locked prior to calling this routine.  We will always
    acquire it here.


    Fcb - Fcb representing this stream.

    FileOffset - Lookup the allocation beginning at this point.

    DiskOffset - Address to store the logical disk offset.

    ByteCount - Address to store the number of contiguous bytes beginning
        at DiskOffset above.

Return Value: