dpdk: Deprecate vhost-user dequeue zero-copy.

Dequeue zero-copy is no longer supported for vhost-user client mode
in DPDK due to commit [1].

In addition to this, zero-copy mode has been proposed to be marked
deprecated in [2] with removal in the next DPDK LTS release.

This commit deprecates support for vhost-user dequeue zero-copy in OVS
with its removal expected in the next OVS release.

[1] 715070ea10e6 ("vhost: prevent zero-copy with incompatible client
    mode")
[2] http://mails.dpdk.org/archives/dev/2020-August/177236.html

Signed-off-by: Ian Stokes <ian.stokes@intel.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Ilya Maximets <i.maximets@ovn.org>

lib/netdev-dpdk.c

@@ -5085,6 +5085,8 @@ netdev_dpdk_vhost_client_reconfigure(struct netdev *netdev)
                       dev->up.name, dev->vhost_id);
             if (zc_enabled) {
                 VLOG_INFO("Zero copy enabled for vHost port %s", dev->up.name);
+                VLOG_WARN("Zero copy support is deprecated and will be "
+                          "removed in the next OVS release.");
             }
         }