nfsd: Add a sanity check to SecinfoNoname for file type

Robert Morris reported that, for the case of SecinfoNoname
with the Parent option, providing a non-directory could
cause a crash.

This patch adds a sanity check for v_type == VDIR for
this case, to avoid the crash.

Reported by: rtm@lcs.mit.edu
PR: 260300
MFC after: 2 weeks