ftrace: Do not process STUB functions in ftrace_ops_list_func()

The function_graph tracer has a stub function and its ops flag has the
FTRACE_OPS_FL_STUB set. As the function graph does not use the
ftrace_ops->func pointer but instead is called by a separate part of the
ftrace trampoline. The function_graph tracer still requires to pass in a
ftrace_ops that may also hold the hash of the functions to call. But there's
no reason to test that hash in the function tracing portion. Instead of
testing to see if we should call the stub function, just test if the ops has
FTRACE_OPS_FL_STUB set, and just skip it.
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -6260,6 +6260,9 @@ __ftrace_ops_list_func(unsigned long ip, unsigned long parent_ip,
 	preempt_disable_notrace();
 
 	do_for_each_ftrace_op(op, ftrace_ops_list) {
+		/* Stub functions don't need to be called nor tested */
+		if (op->flags & FTRACE_OPS_FL_STUB)
+			continue;
 		/*
 		 * Check the following for each ops before calling their func:
 		 *  if RCU flag is set, then rcu_is_watching() must be true