xfstests: make the scratch device for generic/256 slightly larger

[Rich Johnston]

Hey Josef,

Sorry this took so long to commit, I did not see it had been reviewed.

Thanks again for the patch
--Rich

commit 617491caf44d5e93006d774da95f532e9d7de4c4
Author: Josef Bacik <jbacik at fusionio.com>
Date:   Tue Jul 2 19:17:18 2013 +0000