nfsd/sunrpc: abstract out svc_set_num_threads to sv_ops
Add an operation that will do setup of the service. In the case of a
classic thread-based service that means starting up threads. In the case
of a workqueue-based service, the setup will do something different.
Signed-off-by: Shirley Ma <shirley.ma@oracle.com> Acked-by: Jeff Layton <jlayton@primarydata.com> Tested-by: Shirley Ma <shirliey.ma@oracle.com> Signed-off-by: J. Bruce Fields <bfields@redhat.com>