nfsd: remove nfsd4_callback.cb_op

We can always get at the private data by using container_of, no need for
a void pointer.  Also introduce a little to_delegation helper to avoid
opencoding the container_of everywhere.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>