module: ensure that kobject_put() is safe for module type kobjects

In 'lookup_or_create_module_kobject()', an internal kobject is created
using 'module_ktype'. So call to 'kobject_put()' on error handling
path causes an attempt to use an uninitialized completion pointer in
'module_kobject_release()'. In this scenario, we just want to release
kobject without an extra synchronization required for a regular module
unloading process, so adding an extra check whether 'complete()' is
actually required makes 'kobject_put()' safe.

Reported-by: syzbot+7fb8a372e1f6add936dd@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7fb8a372e1f6add936dd
Fixes: 942e443 ("module: Fix mod->mkobj.kobj potentially freed too early")
Cc: stable@vger.kernel.org
Suggested-by: Petr Pavlu <petr.pavlu@suse.com>
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Link: https://lore.kernel.org/r/20250507065044.86529-1-dmantipov@yandex.ru
Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>

kernel/params.c

@@ -943,7 +943,9 @@ struct kset *module_kset;
 static void module_kobj_release(struct kobject *kobj)
 {
 	struct module_kobject *mk = to_module_kobject(kobj);
-	complete(mk->kobj_completion);
+
+	if (mk->kobj_completion)
+		complete(mk->kobj_completion);
 }
 
 const struct kobj_type module_ktype = {