exemptfee: Only apply to the root payment and fix logs

We were applying the fee exemption to all payments individually, which is ok
until we switch to MPP, where amounts change. Also the log entry was referring
to the total amount, and not the fee of the payment.

plugins/libplugin-pay.c

@@ -1661,17 +1661,18 @@ static struct exemptfee_data *exemptfee_data_init(struct payment *p)
 
 static void exemptfee_cb(struct exemptfee_data *d, struct payment *p)
 {
-	if (p->step != PAYMENT_STEP_INITIALIZED)
+	if (p->step != PAYMENT_STEP_INITIALIZED || p->parent != NULL)
 		return payment_continue(p);
 
 	if (amount_msat_greater_eq(d->amount, p->constraints.fee_budget)) {
-		p->constraints.fee_budget = d->amount;
-		p->start_constraints->fee_budget = d->amount;
 		plugin_log(
 		    p->plugin, LOG_INFORM,
-		    "Payment amount is below exemption threshold, "
+		    "Payment fee constraint %s is below exemption threshold, "
 		    "allowing a maximum fee of %s",
-		    type_to_string(tmpctx, struct amount_msat, &p->constraints.fee_budget));
+		    type_to_string(tmpctx, struct amount_msat, &p->constraints.fee_budget),
+		    type_to_string(tmpctx, struct amount_msat, &d->amount));
+		p->constraints.fee_budget = d->amount;
+		p->start_constraints->fee_budget = d->amount;
 	}
 	return payment_continue(p);
 }