RFC: du reports a 1.2PB file on a 1TB btrfs disk

[Jim Meyering]

[note: this was not a sparse file and I wasn't using --apparent-size]

On a 1TB local btrfs file system, I found a log file for which du
reported a size of 1.2 petabytes. Quite surprising for a moment, then
I remembered btrfs's "compression".

Should we teach du to report compressed size? Via a new option? Or
make it report *that* size by default, and add a new option to report
the other number. Or should we just view this as a btrfs+compression
infelicity?

Otherwise, du provides no way of seeing how much of the actual disk
space is being used by such FS-compressed files.