tracing: Remove usage of list iterator variable after the loop

In preparation to limit the scope of a list iterator to the list
traversal loop, use a dedicated pointer to point to the found element
[1].

Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
Link: https://lkml.kernel.org/r/20220427170734.819891-3-jakobkoschel@gmail.com

Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/trace_events.c

@@ -1723,9 +1723,9 @@ static LIST_HEAD(event_subsystems);
 
 static int subsystem_open(struct inode *inode, struct file *filp)
 {
+	struct trace_subsystem_dir *dir = NULL, *iter_dir;
+	struct trace_array *tr = NULL, *iter_tr;
 	struct event_subsystem *system = NULL;
-	struct trace_subsystem_dir *dir = NULL; /* Initialize for gcc */
-	struct trace_array *tr;
 	int ret;
 
 	if (tracing_is_disabled())
@@ -1734,10 +1734,12 @@ static int subsystem_open(struct inode *inode, struct file *filp)
 	/* Make sure the system still exists */
 	mutex_lock(&event_mutex);
 	mutex_lock(&trace_types_lock);
-	list_for_each_entry(tr, &ftrace_trace_arrays, list) {
-		list_for_each_entry(dir, &tr->systems, list) {
-			if (dir == inode->i_private) {
+	list_for_each_entry(iter_tr, &ftrace_trace_arrays, list) {
+		list_for_each_entry(iter_dir, &iter_tr->systems, list) {
+			if (iter_dir == inode->i_private) {
 				/* Don't open systems with no events */
+				tr = iter_tr;
+				dir = iter_dir;
 				if (dir->nr_events) {
 					__get_system_dir(dir);
 					system = dir->subsystem;
@@ -1753,9 +1755,6 @@ static int subsystem_open(struct inode *inode, struct file *filp)
 	if (!system)
 		return -ENODEV;
 
-	/* Some versions of gcc think dir can be uninitialized here */
-	WARN_ON(!dir);
-
 	/* Still need to increment the ref count of the system */
 	if (trace_array_get(tr) < 0) {
 		put_system(dir);