netdev-dpdk: assume dpdkr peer can be multi-producer/consumer

netdev-dpdk: assume dpdkr peer can be multi-producer/consumer

Although netdev does explicit locking, it is only valid from the ovs
perspective, then only the ring ends used by ovs should be declared as
single producer/consumer.
The other ends that are used by the application should be declared as
multiple producer/consumer that is the most general case.

Signed-off-by: Mauricio Vasquez B <mauricio.vasquezbernal@studenti.polito.it>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Acked-by: Daniele Di Proietto <diproiettod@vmware.com>