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]