landlock: Fix d_parent walk

In the Linux kernel, the following vulnerability has been resolved: landlock: Fix d_parent walk The WARN_ON_ONCE() in collect_domain_accesses() can be triggered when trying to link a root mount point. This cannot work in practice because this directory is mounted, but the VFS check is done after the call to security_path_link(). Do not use source directory's d_parent when the source directory is the mount point. [mic: Fix commit message]