put_mnt_ns(): use guards

clean fit; guards can't be weaker due to umount_tree() call.
Setting emptied_ns requires namespace_excl, but not anything
mount_lock-related.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -6153,12 +6153,10 @@ void put_mnt_ns(struct mnt_namespace *ns)
 {
 	if (!refcount_dec_and_test(&ns->ns.count))
 		return;
-	namespace_lock();
+	guard(namespace_excl)();
 	emptied_ns = ns;
-	lock_mount_hash();
+	guard(mount_writer)();
 	umount_tree(ns->root, 0);
-	unlock_mount_hash();
-	namespace_unlock();
 }
 
 struct vfsmount *kern_mount(struct file_system_type *type)