ofproto-dpif-xlate: Prevent duplicating of traffic to a mirror port

ofproto-dpif-xlate: Prevent duplicating of traffic to a mirror port

Currently ofproto design disallow duplicating output packet on forwarding
and mirroring to/from same ovs port. Next scenario reveal lack of design:
1. Send ping between regular ovs ports (VFs, for ex.), stop it.
2. While rule still exist, make mirror for one of the ports.
Prevent duplicating of traffic to a mirror port.

Fixes: 86e2dcddce85 ("dpif-xlate: Snoop multicast packets and send them properly")
Signed-off-by: Dmytro Linkin <dmitrolin@mellanox.com>
Acked-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>