tracing/osnoise: Fix possible recursive locking for cpus_read_lock()

Lockdep reports this deadlock log:

osnoise: could not start sampling thread
============================================
WARNING: possible recursive locking detected
--------------------------------------------
       CPU0
       ----
  lock(cpu_hotplug_lock);
  lock(cpu_hotplug_lock);

 Call Trace:
  <TASK>
  print_deadlock_bug+0x282/0x3c0
  __lock_acquire+0x1610/0x29a0
  lock_acquire+0xcb/0x2d0
  cpus_read_lock+0x49/0x120
  stop_per_cpu_kthreads+0x7/0x60
  start_kthread+0x103/0x120
  osnoise_hotplug_workfn+0x5e/0x90
  process_one_work+0x44f/0xb30
  worker_thread+0x33e/0x5e0
  kthread+0x206/0x3b0
  ret_from_fork+0x31/0x50
  ret_from_fork_asm+0x11/0x20
  </TASK>

This is the deadlock scenario:
osnoise_hotplug_workfn()
  guard(cpus_read_lock)();      // first lock call
  start_kthread(cpu)
    if (IS_ERR(kthread)) {
      stop_per_cpu_kthreads(); {
        cpus_read_lock();      // second lock call. Cause the AA deadlock
      }
    }

It is not necessary to call stop_per_cpu_kthreads() which stops osnoise
kthread for every other CPUs in the system if a failure occurs during
hotplug of a certain CPU.
For start_per_cpu_kthreads(), if the start_kthread() call fails,
this function calls stop_per_cpu_kthreads() to handle the error.
Therefore, similarly, there is no need to call stop_per_cpu_kthreads()
again within start_kthread().
So just remove stop_per_cpu_kthreads() from start_kthread to solve this issue.

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/20250321095249.2739397-1-ranxiaokai627@163.com


Fixes: c8895e27 ("trace/osnoise: Support hotplug operations")
Signed-off-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>