Also accept btrfs tools using IEC prefix multipliers (#706914)

Currently the btrfs command outputs figures to 2 decimal places followed
by an SI multiplier, e.g. 1.00GB.

This patch to btrfs-progs has been included in the integration
repository and will likely be included in the official btrfs-progs
repository at some point.  It changes btrfs-progs to use IEC
multipliers, e.g. 1.00GiB.  In fact multipliers already aren't used for
figures less than 1024.
    [PATCH] btrfs-progs: use IEC units for size
    http://permalink.gmane.org/gmane.comp.file-systems.btrfs/26888
    https://patchwork.kernel.org/patch/2825841/

Make GParted capable of also accepting IEC prefix multipliers, just "B"
for bytes and no multiplier, as well as an optional space between the
number and multiplier.  Therefore accept values like these:
    1.00GB         1.00 GB
    1.00GiB        1.00 GiB
    1073741824B    1073741824 B
    1073741824

Closes Bug #706914 - Prepare for btrfs tools using IEC prefix
                     multipliers