nfsd: Fix handling of the error case for nfsvno_open

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.

Reviewed by: mjg
PR: 268971
Tested by: ish@amail.plala.or.jp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D38430