net/sched: cls_api: Add extack message for unsupported action offload

For better error reporting to user space, add an extack message when the
requested action does not support offload.

Example:

 # echo 1 > /sys/kernel/tracing/events/netlink/netlink_extack/enable

 # tc filter add dev dummy0 ingress pref 1 proto all matchall skip_sw action nat ingress 192.0.2.1 198.51.100.1
 Error: cls_matchall: Failed to setup flow action.
 We have an error talking to the kernel

 # cat /sys/kernel/tracing/trace_pipe
       tc-181     [000] b..1.    88.406093: netlink_extack: msg=Action does not support offload
       tc-181     [000] .....    88.406108: netlink_extack: msg=cls_matchall: Failed to setup flow action
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/cls_api.c

@@ -3517,11 +3517,13 @@ static int tc_setup_offload_act(struct tc_action *act,
 				struct netlink_ext_ack *extack)
 {
 #ifdef CONFIG_NET_CLS_ACT
-	if (act->ops->offload_act_setup)
+	if (act->ops->offload_act_setup) {
 		return act->ops->offload_act_setup(act, entry, index_inc, true,
 						   extack);
-	else
+	} else {
+		NL_SET_ERR_MSG(extack, "Action does not support offload");
 		return -EOPNOTSUPP;
+	}
 #else
 	return 0;
 #endif