NFSv4.x: Set up struct svc_serv_ops for the callback channel

In order to manage the threads using svc_set_num_threads, we need to
fill in a few extra fields.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>