installer: Correct label on volume page

Previously the efi size was used in the label to tell the rootfs size.
This is obviously wrong.
Now the rootfs size is used in the rootfs label.

Change-Id: I45fbbf1ccdd5b4ccc9ed0a72b1ba3b3f8b27263a
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1174150
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>