util: Remove per-buffer header in linear alloc for release mode

util: Remove per-buffer header in linear alloc for release mode

There's only need to keep the offset and size of the latest buffer,
so rename linear_header into linear_ctx and change the code to
keep records there.

For debug mode we still keep a header, now called linear_node_canary,
to have a magic check.  Since due to alignment we have a free space,
also keep the individual occupation of each node (offset), for
debugging.

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