util: Use an opaque type for linear context

util: Use an opaque type for linear context

In the linear allocation only the parent (context) can be used
to allocate new children, so let's use an opaque type to identify
the linear context.  This is similar to what's done in GC allocator.

Update the documentation and a couple of function names to
refer to linear context instead of linear parent.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25280>