file_handle::extents on Windows does not trim the returned list correctly

[ned14]

Discussed in #154

^{Originally posted by tastewar January 10, 2025}
We have a snippet of code that is intended to process sparse holes in files with a loop like this:

   std::vector<llfio::file_handle::extent_pair> ext = GetFileHandle().extents().value();
   uint32_t startBlock = 0, endBlock = 0, oldEndBlock = 0;
   for (auto it = ext.begin(); it != ext.end(); it++)
   {
   }

Seems to work fine on Linux, but in Windows, on a newly allocated 4096 byte file, we are getting back that there are 64 extents in the file. The first one looks correct, but the others all have 0xFFFFFFFF for both the beginning and length of the extent. Is this expected? Should we be ignoring any entries in the vector with 0xFFFFFFFF?

[ned14]

Specifically, bytesout at https://github.com/ned14/llfio/blob/develop/include/llfio/v2.0/detail/impl/windows/file_handle.ipp#L388