sched_ext: Support dumping multiple schedulers and add scheduler identification

sched_ext: Support dumping multiple schedulers and add scheduler identification

Extend scx_dump_state() to support multiple schedulers and improve task
identification in dumps. The function now takes a specific scheduler to
dump and can optionally filter tasks by scheduler.

scx_dump_task() now displays which scheduler each task belongs to, using
"*" to mark tasks owned by the scheduler being dumped. Sub-schedulers
are identified with their level and cgroup ID.

The SysRq-D handler now iterates through all active schedulers under
scx_sched_lock and dumps each one separately. For SysRq-D dumps, only
tasks owned by each scheduler are dumped to avoid redundancy since all
schedulers are being dumped. Error-triggered dumps continue to dump all
tasks since only that specific scheduler is being dumped.

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