sched_ext: Provides a sysfs 'events' to expose core event counters

Add a sysfs entry at /sys/kernel/sched_ext/root/events to expose core
event counters through the files system interface. Each line of the file
shows the event name and its counter value.

In addition, the format of scx_dump_event() is adjusted as the event name
gets longer.

Signed-off-by: Changwoo Min <changwoo@igalia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>