sched_ext: Move event_stats_cpu into scx_sched

sched_ext: Move event_stats_cpu into scx_sched

The event counters are going to become per scheduler instance. Move
event_stats_cpu into scx_sched.

- [__]scx_add_event() are updated to take @sch. While at it, add missing
  parentheses around @cnt expansions.

- scx_read_events() is updated to take @sch.

- scx_bpf_events() accesses scx_root under RCU read lock.

v2: - Replace stale scx_bpf_get_event_stat() reference in a comment with
      scx_bpf_events().

    - Trivial goto label rename.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Acked-by: Changwoo Min <changwoo@igalia.com>