[LTP] [PATCH v2 2/2] Fix calculations and defined constants

[Petr Vorel]

Hi Jozef, Martin,

I like this version and fixes, it speeds up nearly to the original version,
even on ppc64le, thus merged (with slightly adjusted commit message).

f_bavail change could have been separated, but merged squashed.

Thanks for working on this!

Kind regards,
Petr