Fix a panic that occurs when vrele() is called with a NULL ni_dvp

[rmacklem]

Summary

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.)

Test Plan

Tested by the reporter of PR#268828.

[mjg]

thank you and sorry for the delay!