gfile: Fix file size detection when copying on btrfs

When the `g_file_copy` function is used with files on BTRFS, the
`GLib-GIO-FATAL-CRITICAL: GFileInfo created without standard::size`
error is printed. This is because the `g_file_get_size` function
is used to obtain the file size for the progress callback, but it uses
the wrong `GFileInfo` object that is meant for attributes to be copied
with the file. The file size attribute is missing there obviously. Let's
obtain the file size over the `fstat` call the same way as it is done in
the `splice_stream_with_progress` function to get rid of those errors
and to fix the progress reporting.