nfs_clvnops.c: Fix the case where va_flags are being cleared

Commits c5d72d2 and 3b6d4c6 broke the case where the
archive/hidden/system attributes are being set false
(UF_ARCHIVE, UF_HIDDEN or UF_SYSTEM bits being cleared.)
and the NFS server does not support those attributes.
These patches only checked for support if the
archive/hidden/system attributes were non-zero.

This patch fixes the problem.

PR: 296088
Tested by: Joshua Kinard <freebsd@kumba.dev>
MFC after: 1 week
Fixes: c5d72d29fe0e ("nfsv4: Add support for the NFSv4 hidden and system attributes")