tracing/events: Fix to check the simple_tsk_fn creation

Sashiko pointed that this sample code does not correctly handle the
failure of thread creation because kthread_run() can return -errno.

Check the simple_tsk_fn is correctly initialized (created) or not.

Link: https://lore.kernel.org/all/178165817322.269421.3992299509400184196.stgit@devnote2/

Link: https://sashiko.dev/#/patchset/178092865666.163648.10457567771536160909.stgit%40devnote2

Fixes: 9cfe06f8cd ("tracing/events: add trace-events-sample")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

samples/trace_events/trace-events-sample.c

@@ -107,6 +107,10 @@ int foo_bar_reg(void)
 	 * for consistency sake, we still take the thread_mutex.
 	 */
 	simple_tsk_fn = kthread_run(simple_thread_fn, NULL, "event-sample-fn");
+	if (IS_ERR_OR_NULL(simple_tsk_fn)) {
+		pr_err("Failed to create simple_thread_fn\n");
+		simple_tsk_fn = NULL;
+	}
  out:
 	mutex_unlock(&thread_mutex);
 	return 0;