tracing/timerlat: Fix duplicated kthread creation due to CPU online/offline

tracing/timerlat: Fix duplicated kthread creation due to CPU online/offline

osnoise_hotplug_workfn() is the asynchronous online callback for
"trace/osnoise:online". It may be congested when a CPU goes online and
offline repeatedly and is invoked for multiple times after a certain
online.

This will lead to kthread leak and timer corruption. Add a check
in start_kthread() to prevent this situation.

Cc: [email protected]
Cc: Masami Hiramatsu <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Link: https://lore.kernel.org/[email protected]
Fixes: c8895e271f79 ("trace/osnoise: Support hotplug operations")
Signed-off-by: Wei Li <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>