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>

drivers/base/class.c

@@ -127,7 +127,7 @@ static const struct kobj_type class_ktype = {
 };
 
 int class_create_file_ns(const struct class *cls, const struct class_attribute *attr,
-			 const void *ns)
+			 const struct ns_common *ns)
 {
 	struct subsys_private *sp = class_to_subsys(cls);
 	int error;
@@ -143,7 +143,7 @@ int class_create_file_ns(const struct class *cls, const struct class_attribute *
 EXPORT_SYMBOL_GPL(class_create_file_ns);
 
 void class_remove_file_ns(const struct class *cls, const struct class_attribute *attr,
-			  const void *ns)
+			  const struct ns_common *ns)
 {
 	struct subsys_private *sp = class_to_subsys(cls);