Fix a crash in the NFSv4 server.

The PR reported a crash that occurred when a file was removed while
client(s) were actively doing lock operations on it.
Since nfsvno_getvp() will return NULL when the file does not exist,
the bug was obvious and easy to fix via this patch. It is a little
surprising that this wasn't found sooner, but I guess the above
case rarely occurs.

Tested by: iron.udjin@gmail.com
PR: 242768
Reported by: iron.udjin@gmail.com
MFC after: 2 weeks