net/sched: act_tunnel_key: Allow key-less tunnels

Allow setting a tunnel without a tunnel key. This is required for
tunneling protocols, such as GRE, that define the key as an optional
field.

Signed-off-by: Adi Nissim <adin@mellanox.com>
Acked-by: Or Gerlitz <ogerlitz@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>