mm: kfence: fix using kfence_metadata without initialization in show_object()

The variable kfence_metadata is initialized in kfence_init_pool(), then,
it is not initialized if kfence is disabled after booting.  In this case,
kfence_metadata will be used (e.g.  ->lock and ->state fields) without
initialization when reading /sys/kernel/debug/kfence/objects.  There will
be a warning if you enable CONFIG_DEBUG_SPINLOCK.  Fix it by creating
debugfs files when necessary.

Link: https://lkml.kernel.org/r/20230315034441.44321-1-songmuchun@bytedance.com
Fixes: 0ce20dd8 ("mm: add Kernel Electric-Fence infrastructure")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Tested-by: Marco Elver <elver@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: SeongJae Park <sjpark@amazon.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/kfence/core.c

@@ -726,10 +726,14 @@ static const struct seq_operations objects_sops = {
 };
 DEFINE_SEQ_ATTRIBUTE(objects);
 
-static int __init kfence_debugfs_init(void)
+static int kfence_debugfs_init(void)
 {
-	struct dentry *kfence_dir = debugfs_create_dir("kfence", NULL);
+	struct dentry *kfence_dir;
 
+	if (!READ_ONCE(kfence_enabled))
+		return 0;
+
+	kfence_dir = debugfs_create_dir("kfence", NULL);
 	debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
 	debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
 	return 0;
@@ -883,6 +887,8 @@ static int kfence_init_late(void)
 	}
 
 	kfence_init_enable();
+	kfence_debugfs_init();
+
 	return 0;
 }