Wrong disk-size displayed when using btrfs

[Andrew]

The disk size displayed when clicking on 'Details' is roughly double the size of my actual disk. I noticed the same thing happening my arch linux machine which was also using btrfs.

Description: Ubuntu 15.04
Release: 15.04

unity-control-center:
  Installed: 15.04.0+15.04.20150410-0ubuntu1
  Candidate: 15.04.0+15.04.20150410-0ubuntu1
  Version table:
 *** 15.04.0+15.04.20150410-0ubuntu1 0
        500 http://us.archive.ubuntu.com/ubuntu/ vivid/main amd64 Packages
        100 /var/lib/dpkg/status

What I expected to happen:
"Details" should show the correct file size available.

What happened instead:
"Details" shows double the disk size.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: unity-control-center 15.04.0+15.04.20150410-0ubuntu1
ProcVersionSignature: Ubuntu 3.19.0-15.15-generic 3.19.3
Uname: Linux 3.19.0-15-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.17.2-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Sun Apr 26 18:57:36 2015
ExecutablePath: /usr/bin/unity-control-center
InstallationDate: Installed on 2015-04-26 (0 days ago)
InstallationMedia: It
ProcEnviron:
 LANGUAGE=en_US
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: unity-control-center
UpgradeStatus: No upgrade log present (probably fresh install)
usr_lib_unity-control-center: deja-dup 32.0-0ubuntu5

[Sebastien Bacher (seb128)]

Seems similar to https://bugzilla.gnome.org/show_bug.cgi?id=708786