Use f_bavail to calculate free space

Failures are seen on devices with
Linux 3.10. And they are mainly due to this change:
https://lwn.net/Articles/546473/
The blocks reserved in this change is not the same thing as what we
think are reserved for common usage of root user. And this part is
included in free blocks but not in available blocks.

Change-Id: Ib29e12d775b86ef657c0af7fa7a944d2b1e12dc8