ns: pad refcount

Note no effort is made to make sure structs embedding the namespace are
themselves aligned, so this is not guaranteed to eliminate cacheline
bouncing due to refcount management.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251203092851.287617-2-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: mjguzik

include/linux/ns/ns_common_types.h

@@ -108,11 +108,13 @@ extern const struct proc_ns_operations utsns_operations;
  * @ns_tree: namespace tree nodes and active reference count
  */
 struct ns_common {
+	struct {
+		refcount_t __ns_ref; /* do not use directly */
+	} ____cacheline_aligned_in_smp;
 	u32 ns_type;
 	struct dentry *stashed;
 	const struct proc_ns_operations *ops;
 	unsigned int inum;
-	refcount_t __ns_ref; /* do not use directly */
 	union {
 		struct ns_tree;
 		struct rcu_head ns_rcu;