util-linux: util-linux: X-mount.subdir detached-tree resolution can escape via intermediate symlinks

The X-mount.subdir option uses a detached-tree fast path on Linux 6.15 and later and passes the configured subdirectory to opentree with ATSYMLINKNOFOLLOW. That flag does not stop intermediate symlink traversal or keep resolution inside the newly mounted filesystem. A local unprivileged user with...