sched/debug: Dump domains' level

Knowing domain's level exactly can be useful when setting
relax_domain_level or cpuset.sched_relax_domain_level

Usage:

  cat /debug/sched/domains/cpu0/domain1/level

to dump cpu0 domain1's level.

SDM macro is not used because sd->level is 'int' and
it would hide the type mismatch between 'int' and 'u32'.

Signed-off-by: Vitalii Bursov <vitaly@bursov.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Acked-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Link: https://lore.kernel.org/r/9489b6475f6dd6fbc67c617752d4216fa094da53.1714488502.git.vitaly@bursov.com