fgraph: Keep track of when fgraph_ops are registered or not

Add a warning if unregister_ftrace_graph() is called without ever
registering it, or if register_ftrace_graph() is called twice. This can
detect errors when they happen and not later when there's a side effect:

Link: https://lore.kernel.org/all/[e-mail]/

Cc: Masami Hiramatsu <[e-mail]>
Cc: Mathieu Desnoyers <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Signed-off-by: Steven Rostedt (Google) <[e-mail]>

kernel/trace/fgraph.c

···
 	int ret = 0;
 	int i = -1;
 
+	if (WARN_ONCE(gops->ops.flags & FTRACE_OPS_FL_GRAPH,
+		      "function graph ops registered again"))
+		return -EBUSY;
+
 	guard(mutex)(&ftrace_lock);
 
 	if (!fgraph_stack_cachep) {
···
 {
 	int command = 0;
 
+	if (WARN_ONCE(!(gops->ops.flags & FTRACE_OPS_FL_GRAPH),
+		      "function graph ops unregistered without registering"))
+		return;
+
 	guard(mutex)(&ftrace_lock);
 
 	if (unlikely(!ftrace_graph_active))
-		return;
+		goto out;
 
 	if (unlikely(gops->idx < 0 || gops->idx >= FGRAPH_ARRAY_SIZE ||
 		     fgraph_array[gops->idx] != gops))
-		return;
+		goto out;
 
 	if (fgraph_lru_release_index(gops->idx) < 0)
-		return;
+		goto out;
 
 	fgraph_array[gops->idx] = &fgraph_stub;
 
···
 		unregister_trace_sched_switch(ftrace_graph_probe_sched_switch, NULL);
 	}
 	gops->saved_func = NULL;
+ out:
+	gops->ops.flags &= ~FTRACE_OPS_FL_GRAPH;
 }