ofproto-dpif assumed that the datapath initialized the tun_id of a
flow on egress, to its tun_id on ingress. For this reason, if
OpenFlow actions set the tun_id to a flow's ingress tun_id,
ofproto-dpif would fail to emit a set_tunnel action.
Reported-by: Igor Ganichev <iganichev@nicira.com> Reported-by: Pankaj Thakkar <thakkar@nicira.com>