fix crashes caused by not setting done_namei correctly

[rmacklem]

Summary

Using done_namei instead of ni_startdir did not
fix the crashes reported in the PR. Upon looking
more closely at the code, the only case where the
code near the end of nfsvno_open() needs to be
executed is when nfsvno_namei() has succeeded,
but a subsequent error was detected.

This patch uses done_namei to indicate this case.

Also, nfsvno_relpathbuf() should only be called for
this case and not whenever nfsvno_open() is called
with nd_repstat != 0. A bug was introduced here when
the HASBUF flag was deleted.

Test Plan

Tested by the reporter of the PR.

[mjg]

style suggests keeping explicit NULL comparisons, but i don't think it matters

[rmacklem]

Yep. And style also states that the style of a function
(or is it a file?) should not be changed unless you are
re-writing 50% of it.

In this case, I put the code back in the stable/13 form,
because I figured it would be less confusing if someone
compares it.