NFSv4: Don't use cred->cr_ops->cr_name in nfs4_proc_setclientid()

With the recent change to generic creds, we can no longer use
cred->cr_ops->cr_name to distinguish between RPCSEC_GSS principals and
AUTH_SYS/AUTH_NULL identities. Replace it with the rpc_authops->au_name
instead...

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>