tracing: Add rcu annotation around file->filter accesses

Running sparse on trace_events_filter.c triggered several warnings about
file->filter being accessed directly even though it's annotated with __rcu.

Add rcu_dereference() around it and shuffle the logic slightly so that
it's always referenced via accessor functions.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/20250607102821.6c7effbf@gandalf.local.home
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: rostedt

kernel/trace/trace_events_filter.c

@@ -1250,7 +1250,9 @@ static void append_filter_err(struct trace_array *tr,
 
 static inline struct event_filter *event_filter(struct trace_event_file *file)
 {
-	return file->filter;
+	return rcu_dereference_protected(file->filter,
+					 lockdep_is_held(&event_mutex));
+
 }
 
 /* caller must hold event_mutex */
@@ -1320,7 +1322,7 @@ void free_event_filter(struct event_filter *filter)
 static inline void __remove_filter(struct trace_event_file *file)
 {
 	filter_disable(file);
-	remove_filter_string(file->filter);
+	remove_filter_string(event_filter(file));
 }
 
 static void filter_free_subsystem_preds(struct trace_subsystem_dir *dir,
@@ -1405,7 +1407,7 @@ static void try_delay_free_filter(struct event_filter *filter)
 
 static inline void __free_subsystem_filter(struct trace_event_file *file)
 {
-	__free_filter(file->filter);
+	__free_filter(event_filter(file));
 	file->filter = NULL;
 }
 
@@ -1465,7 +1467,7 @@ static void filter_free_subsystem_filters(struct trace_subsystem_dir *dir,
 	list_for_each_entry(file, &tr->events, list) {
 		if (file->system != dir || !file->filter)
 			continue;
-		__free_filter(file->filter);
+		__free_subsystem_filter(file);
 	}
 	__free_filter(filter);
 }