staging: zsmalloc: rename zspage_order with zspage_pages
zspage_order defines how many pages are needed to make a zspage.
So _order_ is rather awkward naming. It already deceive Jonathan
- http://lwn.net/Articles/477067/
" For each size, the code calculates an optimum number of pages (up to 16)"
Let's change from _order_ to _pages_ and some function names.