make the blkcg and blkcg structures private

[Christoph Hellwig]

On Thu, Apr 21, 2022 at 11:44:43AM -1000, Tejun Heo wrote:
> The patches look all good to me and I'm not against making things more
> private but can you elaborate on the rationale a bit more? By and large, we
> have never been shy about putting things in the headers if there's *any*
> (perceived) gain to be made from doing so, or even just as a way to pick the
> locations for different things - type defs go on header and so on. Most of
> the inlines and [un]likely's that we have are rather silly with modern
> compilers with global optimizations, so it does make sense to get tidier,
> but if that's the rationale, mentioning that in the commit message, even
> briefly, would be great - ie. it should explain the benefits of adding these
> few accessors to keep the definition private.

Mostly to help me understand the code :)  between all the moving to
and from the css struture it is a bit of a mess, and limiting the scope
that deals with the structures greatly helps with that.