function-graph: always initialize task ret_stack

On creating a new task while running the function graph tracer, if
we fail to allocate the ret_stack, and then fail the fork, the
code will free the parent ret_stack. This is because the child
duplicated the parent and currently points to the parent's ret_stack.

This patch always initializes the task's ret_stack to NULL.

[ Impact: prevent crash of parent on low memory during fork ]
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -2738,6 +2738,9 @@ void unregister_ftrace_graph(void)
 /* Allocate a return stack for newly created task */
 void ftrace_graph_init_task(struct task_struct *t)
 {
+	/* Make sure we do not use the parent ret_stack */
+	t->ret_stack = NULL;
+
 	if (atomic_read(&ftrace_graph_active)) {
 		struct ftrace_ret_stack *ret_stack;
 
@@ -2753,8 +2756,7 @@ void ftrace_graph_init_task(struct task_struct *t)
 		/* make curr_ret_stack visable before we add the ret_stack */
 		smp_wmb();
 		t->ret_stack = ret_stack;
-	} else
-		t->ret_stack = NULL;
+	}
 }
 
 void ftrace_graph_exit_task(struct task_struct *t)