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

For better error reporting to user space, add an extack message when
vlan 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 the current
set of 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_vlan.c

@@ -399,6 +399,7 @@ static int tcf_vlan_offload_act_setup(struct tc_action *act, void *entry_data,
 			tcf_vlan_push_eth(entry->vlan_push_eth.src, entry->vlan_push_eth.dst, act);
 			break;
 		default:
+			NL_SET_ERR_MSG_MOD(extack, "Unsupported vlan action mode offload");
 			return -EOPNOTSUPP;
 		}
 		*index_inc = 1;