tracing/osnoise: Use for_each_online_cpu() instead of for_each_cpu()

Replace the opencoded for_each_cpu(cpu, cpu_online_mask) loop with the
more readable and equivalent for_each_online_cpu(cpu) macro.

Link: https://lore.kernel.org/20250811064158.2456-1-wangfushuai@baidu.com


Signed-off-by: Fushuai Wang <wangfushuai@baidu.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>