net/sched: act_tunnel_key: Don't dump dst port if it wasn't set

It's possible to set a tunnel without a destination port. However,
on dump(), a zero dst port is returned to user space even if it was not
set, fix that.

Note that so far it wasn't required, b/c key less tunnels were not
supported and the UDP tunnels do require destination port.

Signed-off-by: Adi Nissim <adin@mellanox.com>
Reviewed-by: Oz Shlomo <ozsh@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>