nfsserver: Fix vrele() panic in nfsvno_open()

Commit 65127e982b94 removed a check for ni_startdir != NULL.
This allowed the vrele(ndp->ni_dvp) to be called with
a NULL argument.

This patch adds a new boolean argument to nfsvno_open()
that can be checked instead of ni_startdir, since mjg@ requested
that ni_startdir not be used. (Discussed in PR#268828.)

PR: 268828
Reviewed by: mjg
Differential Revision: https://reviews.freebsd.org/D38032