nfs4: Make the v4 callback service hidden

To avoid hangs in the svc_unregister(), on version 4 mounts
(and unmounts), when rpcbind is not running, make the nfs4 callback
program an 'hidden' service by setting the 'vs_hidden' flag in the
nfs4_callback_version structure.

Signed-off-by: Steve Dickson <steved@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>