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

For better error reporting to user space, add an extack message when
mirred 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 ingress/egress
mirror/redirect.
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_mirred.c

@@ -479,6 +479,7 @@ static int tcf_mirred_offload_act_setup(struct tc_action *act, void *entry_data,
 			entry->id = FLOW_ACTION_MIRRED_INGRESS;
 			tcf_offload_mirred_get_dev(entry, act);
 		} else {
+			NL_SET_ERR_MSG_MOD(extack, "Unsupported mirred offload");
 			return -EOPNOTSUPP;
 		}
 		*index_inc = 1;