namei: Treat non-tied KLDs as if they had INVARIANTS enabled

When working with a vendor to debug their kernel module,
I found that a non-tied kld which uses NDINIT will panic
due to "namei: bad debugflags " on a kernel compiled with
INVARIANTS because non-tied KLDs do not pick up the
initialization that is done in NDINIT_DBG/NDREINIT_DBG().

Fix this by making this initialization happen for non-KLD_TIED
as well as INVARIANTS

Reviewed by: mjg
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D34588