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

[gallatin]

Summary

When working with a vendor to debug their kernel module, I found that a non-tied kld 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

[mjg]

this needs a comment what the hack is for, as in how come this is not picked up automatically

[gallatin]

• Added a comment to why we enable debug stuff here as per mjg's request