sock: permit SO_ZEROCOPY on PF_RDS socket

allow the application to set SO_ZEROCOPY on the underlying sk
of a PF_RDS socket

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>