net/sched: act_tunnel_key: Add extack message for offload failure

For better error reporting to user space, add an extack message when
tunnel_key action offload fails.

Currently, the failure cannot be triggered, but add a message in case
the action is extended in the future to support more than set/release
modes.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/act_tunnel_key.c

@@ -824,6 +824,7 @@ static int tcf_tunnel_key_offload_act_setup(struct tc_action *act,
 		} else if (is_tcf_tunnel_release(act)) {
 			entry->id = FLOW_ACTION_TUNNEL_DECAP;
 		} else {
+			NL_SET_ERR_MSG_MOD(extack, "Unsupported tunnel key mode offload");
 			return -EOPNOTSUPP;
 		}
 		*index_inc = 1;