fs: use boolean to indicate anonymous mount namespace

Stop playing games with the namespace id and use a boolean instead:

* This will remove the special-casing we need to do everywhere for mount
  namespaces.

* It will allow us to use asserts on the namespace id for initial
  namespaces everywhere.

* It will allow us to put anonymous mount namespaces on the namespaces
  trees in the future and thus make them available to statmount() and
  listmount().

Link: https://patch.msgid.link/20251110-work-namespace-nstree-fixes-v1-10-e8a9264e0fb9@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

fs/mount.h

@@ -27,6 +27,7 @@ struct mnt_namespace {
 	unsigned int		nr_mounts; /* # of mounts in the namespace */
 	unsigned int		pending_mounts;
 	refcount_t		passive; /* number references not pinning @mounts */
+	bool			is_anon;
 } __randomize_layout;
 
 struct mnt_pcp {
@@ -175,7 +176,7 @@ static inline bool is_local_mountpoint(const struct dentry *dentry)
 
 static inline bool is_anon_ns(struct mnt_namespace *ns)
 {
-	return ns->ns.ns_id == 0;
+	return ns->is_anon;
 }
 
 static inline bool anon_ns_root(const struct mount *m)

fs/namespace.c

@@ -4093,8 +4093,9 @@ static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *user_ns, bool a
 		dec_mnt_namespaces(ucounts);
 		return ERR_PTR(ret);
 	}
-	if (!anon)
-		ns_tree_gen_id(new_ns);
+	ns_tree_gen_id(new_ns);
+
+	new_ns->is_anon = anon;
 	refcount_set(&new_ns->passive, 1);
 	new_ns->mounts = RB_ROOT;
 	init_waitqueue_head(&new_ns->poll);