nfsd: Add a KASSERT in nfsvno_open

Commit ded5f2954e1a defined done_namei to indicate that
nd_repstat was set after a successful nfsvno_namei(),
so that a cleanup needs to be done in nfsvno_open().
This only happens when nfsvno_namei() is done with CREATE.

This patch adds a KASSERT() to check for that.

PR: 268971