sched_ext: scx_qmap: move globals and cpu_ctx into a BPF arena map

sched_ext: scx_qmap: move globals and cpu_ctx into a BPF arena map

Arena simplifies verification and allows more natural programming.
Convert scx_qmap to arena as preparation for further sub-sched work.

Move scheduler state from BSS globals and a percpu array map
into a single BPF arena map. A shared struct qmap_arena is declared as
an __arena global so BPF accesses it directly and userspace reaches it
through skel->arena->qa.

Scheduling logic unchanged; only memory backing changes.

v2: Drop "mutable" from comments.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>