nfs_commonsubs.c: Add a sanity check for nid_ngroup

[rmacklem]

Summary

The nfsuserd(8) daemon passes user credentials
(uid + gids) into the kernel for users and groups
identified by name (received from a NFSv4 server).

This patch add a sanity check for the number of
groups (nid_ngroup) passed in.

It's only purpose is to protect against a bogus
nfsuserd(8) running in a jail.

Test Plan

Only tested for a valid working nfsuserd(8) daemon.