build of vfs_lookup.c now broken in non-INVARIANTS kernels

From: Gary Jennejohn <garyj_at_gmx.de>
Date: Sat, 17 Sep 2022 10:41:25 UTC
Compiling vfs_lookup.c now fails when NONINVARIANTS is not included in
the kernel config file because NDVALIDATE is defined as NDVALIDATE_impl,
which itself is only defined when NONINVARIANTS is also defined.

This breaks buildkernel.

--
Gary Jennejohn