sched_ext: Put event_stats_cpu in struct scx_sched_pcpu

scx_sched.event_stats_cpu is the percpu counters that are used to track
stats. Introduce struct scx_sched_pcpu and move the counters inside. This
will ease adding more per-cpu fields. No functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>