nfsd/sunrpc: move sv_function into sv_ops

Since we now have a container for holding svc_serv operations, move the
sv_function into it as well.

Signed-off-by: Shirley Ma <shirley.ma@oracle.com>
Acked-by: Jeff Layton <jlayton@primarydata.com>
Tested-by: Shirley Ma <shirley.ma@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>