datapath: Avoid random tun_id in odp_packet_cmd_execute().

datapath: Avoid random tun_id in odp_packet_cmd_execute().

flow_extract() sets key->tun_id from OVS_CB(packet)->tun_id, which until
now has contained whatever Netlink put there in its NETLINK_CB structure.
Zero it earlier so that its value is at least predictable.

The resulting code is still not correct, because key->tun_id and
key->in_port are now set to arbitrary values.  I have known about this
since I wrote this function (and before, too, in its earlier incarnations),
but until now I did not think that it was a problem because I did not
think that there were any users along this code path.  But that is wrong:
sFlow sampling uses tun_id and in_port and ODP_ACTION_ATTR_CONTROLLER uses
in_port.  So we need a way to pass these back down from userspace.  An
upcoming commit will add a way.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>