nfs, rpc: Ensure kernel credentials have at least one group

This fixes several bugs where some 'struct ucred' in the kernel,
constructed from user input (via nmount(2)) or obtained from other
servers (e.g., gssd(8)), could have an unfilled 'cr_groups' field and
whose 'cr_groups[0]' (or 'cr_gid', which is an alias) was later
accessed, causing an uninitialized access giving random access rights.

Use crsetgroups_fallback() to enforce a fallback group when possible.
For NFS, the chosen fallback group is that of the NFS server in the
current VNET (NFSD_VNET(nfsrv_defaultgid)).

There does not seem to be any sensible fallback available in rpc code
(sys/rpc/svc_auth.c, svc_getcred()) on AUTH_UNIX (TLS or not), so just
fail credential retrieval there. Stock NSS sources, rpc.tlsservd(8) or
rpc.tlsclntd(8) provide non-empty group lists, so will not be impacted.

Discussed with: rmacklem (by mail)
Approved by: markj (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D46918