tracing/events: Fix to check the simple_tsk_fn creation

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: 9cfe06f8cd5c ("tracing/events: add trace-events-sample")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>