Prevent /b from having 5% reserved space
Prevent /b from having 5% reserved space

See http://askubuntu.com/questions/249387/df-h-used-space-avail-free-space-is-less-than-the-total-size-of-home

BUG=skia:609478

Review-Url: https://codereview.chromium.org/2046553002

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot