[modules/disk] Use bavail instead of bfree

[czert]

The ext4 filesystem reserves 5 % of disk size. os.statvfs(path).f_bfree reports this reserve as free disk space, while f_bavail considers it "full".

I believe f_bavail gives more useful information, as the reserved space cannot be used. Also, it's in line with what df reports.

[tobi-wan-kenobi]

Sounds sensible, many thanks!