kernfs: pass struct ns_common instead of const void * for namespace tags

kernfs has historically used const void * to pass around namespace tags
used for directory-level namespace filtering. The only current user of
this is sysfs network namespace tagging where struct net pointers are
cast to void *.

Replace all const void * namespace parameters with const struct
ns_common * throughout the kernfs, sysfs, and kobject namespace layers.
This includes the kobj_ns_type_operations callbacks, kobject_namespace(),
and all sysfs/kernfs APIs that accept or return namespace tags.

Passing struct ns_common is needed because various codepaths require
access to the underlying namespace. A struct ns_common can always be
converted back to the concrete namespace type (e.g., struct net) via
container_of() or to_ns_common() in the reverse direction.

This is a preparatory change for switching to ns_id-based directory
iteration to prevent a KASLR pointer leak through the current use of
raw namespace pointers as hash seeds and comparison keys.

Signed-off-by: Christian Brauner <brauner@kernel.org>

lib/kobject.c

@@ -27,7 +27,7 @@
  * and thus @kobj should have a namespace tag associated with it.  Returns
  * %NULL otherwise.
  */
-const void *kobject_namespace(const struct kobject *kobj)
+const struct ns_common *kobject_namespace(const struct kobject *kobj)
 {
 	const struct kobj_ns_type_operations *ns_ops = kobj_ns_ops(kobj);
 
@@ -1083,9 +1083,9 @@ bool kobj_ns_current_may_mount(enum kobj_ns_type type)
 	return may_mount;
 }
 
-void *kobj_ns_grab_current(enum kobj_ns_type type)
+struct ns_common *kobj_ns_grab_current(enum kobj_ns_type type)
 {
-	void *ns = NULL;
+	struct ns_common *ns = NULL;
 
 	spin_lock(&kobj_ns_type_lock);
 	if (kobj_ns_type_is_valid(type) && kobj_ns_ops_tbl[type])
@@ -1096,7 +1096,7 @@ void *kobj_ns_grab_current(enum kobj_ns_type type)
 }
 EXPORT_SYMBOL_GPL(kobj_ns_grab_current);
 
-void kobj_ns_drop(enum kobj_ns_type type, void *ns)
+void kobj_ns_drop(enum kobj_ns_type type, struct ns_common *ns)
 {
 	spin_lock(&kobj_ns_type_lock);
 	if (kobj_ns_type_is_valid(type) &&