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

For better error reporting to user space, add an extack message when
pedit 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/add
commands.
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_pedit.c

@@ -504,6 +504,7 @@ static int tcf_pedit_offload_act_setup(struct tc_action *act, void *entry_data,
 				entry->id = FLOW_ACTION_ADD;
 				break;
 			default:
+				NL_SET_ERR_MSG_MOD(extack, "Unsupported pedit command offload");
 				return -EOPNOTSUPP;
 			}
 			entry->mangle.htype = tcf_pedit_htype(act, k);