Fix comments: s/granuality/granularity/

I was grepping through the code and some `grep ganularity -R .` didn't
catch what I thought. Then looking closer I saw the term "granuality"
used in only four places (in comments) and granularity in many more
places describing the same idea. Some other facts:

dictionary.com does not know such a word
define:granuality on google is not found (and pages for granuality are
mostly related to patches to the kernel)
it has not been discussed as a term on LKML, AFAICS (=Can Search)

To be consistent, I think granularity should be used everywhere.

Signed-off-by: Kalin KOZHUHAROV <kalin@thinrope.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>