nstree: move nstree types into separate header

Introduce two new fundamental data structures for namespace tree
management in a separate header file.

Link: https://patch.msgid.link/20251110-work-namespace-nstree-fixes-v1-3-e8a9264e0fb9@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/linux/ns/nstree_types.h

@@ -0,0 +1,36 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (c) 2025 Christian Brauner <brauner@kernel.org> */
+#ifndef _LINUX_NSTREE_TYPES_H
+#define _LINUX_NSTREE_TYPES_H
+
+#include <linux/rbtree.h>
+#include <linux/list.h>
+
+/**
+ * struct ns_tree_root - Root of a namespace tree
+ * @ns_rb: Red-black tree root for efficient lookups
+ * @ns_list_head: List head for sequential iteration
+ *
+ * Each namespace tree maintains both an rbtree (for O(log n) lookups)
+ * and a list (for efficient sequential iteration). The list is kept in
+ * the same sorted order as the rbtree.
+ */
+struct ns_tree_root {
+	struct rb_root ns_rb;
+	struct list_head ns_list_head;
+};
+
+/**
+ * struct ns_tree_node - Node in a namespace tree
+ * @ns_node: Red-black tree node
+ * @ns_list_entry: List entry for sequential iteration
+ *
+ * Represents a namespace's position in a tree. Each namespace has
+ * multiple tree nodes for different trees (unified, per-type, owner).
+ */
+struct ns_tree_node {
+	struct rb_node ns_node;
+	struct list_head ns_list_entry;
+};
+
+#endif /* _LINUX_NSTREE_TYPES_H */

include/linux/nstree.h

@@ -3,6 +3,7 @@
 #ifndef _LINUX_NSTREE_H
 #define _LINUX_NSTREE_H
 
+#include <linux/ns/nstree_types.h>
 #include <linux/nsproxy.h>
 #include <linux/rbtree.h>
 #include <linux/seqlock.h>