nfs: Fallback to GID_NOGROUP on no groups

We cannot unconditionally access nfsd's VNET variables in
'sys/kern/vfs_export.c' nor 'sys/fs/nfsserver/nfs_nfsdsubs.c', as they
may not have been compiled in depending on build options.

So, forget about the extra mile of using the configured default group
and use the hardcoded GID_NOGROUP (which differs only on systems running
nfsuserd(8) and with a non-default GID for their "nogroup" group).

Reported by: rpokala, bapt (MINIMAL compile breakup)
Reported by: cy, David Wolfskill (panics caused by mountd(8))
Approved by: markj (mentor)
Fixes: cfbe7a62dc62 ("nfs, rpc: Ensure kernel credentials have at least one group")