samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify

samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify

ftrace_direct_init() assigns kthread_run()'s return value to simple_tsk
without an IS_ERR() check. When kthread_run() fails it returns
ERR_PTR(-ENOMEM), but init still returns 0, so the module loads with
simple_tsk holding an error pointer. On unload, ftrace_direct_exit()
then passes that ERR_PTR to kthread_stop(), leading to a
null-pointer-dereference.

Check the return value of kthread_run() with IS_ERR(); on failure,
unregister the ftrace direct call and propagate the error code.

Link: https://patch.msgid.link/20260826015034.10755-1-vulab@iscas.ac.cn
Fixes: ae0cc3b7e7f5 ("ftrace/samples: Add a sample module that implements modify_ftrace_direct()")
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>