Fix free space calculation

Use fundamental block size to calculate free byte count.
Preferred blocks are not guaranteed to line up with real filesystem blocks.
f_bavail is measured in real filesystem blocks.

(In my case the current calculation is off by a factor of 8, resulting in the free space calculated as being quite a bit larger than the total possible disk space in that directory. However, the off-ness can vary wildly between different hardware and software setups.)