htlcswitch: also set packet amount on modified forward

This fixes an issue where the switch's forwarding logic would think the
bandwidth to forward an HTLC was insufficient for a custom channel HTLC,
because we only overwrote the HTLC's amount and not the packet's (which
is just a short cut struct member anyway).

Author: guggero

htlcswitch/interceptable_switch.go

@@ -658,6 +658,7 @@ func (f *interceptedForward) ResumeModified(
 	switch htlc := f.packet.htlc.(type) {
 	case *lnwire.UpdateAddHTLC:
 		outgoingAmountMsat.WhenSome(func(amount lnwire.MilliSatoshi) {
+			f.packet.amount = amount
 			htlc.Amount = amount
 		})
 

itest/lnd_forward_interceptor_test.go

@@ -407,7 +407,7 @@ func testForwardInterceptorModifiedHtlc(ht *lntest.HarnessTest) {
 	customRecords[crKey] = crValue
 
 	action := routerrpc.ResolveHoldForwardAction_RESUME_MODIFIED
-	newOutgoingAmountMsat := packet.OutgoingAmountMsat + 4000
+	newOutgoingAmountMsat := packet.OutgoingAmountMsat
 
 	err := bobInterceptor.Send(&routerrpc.ForwardHtlcInterceptResponse{
 		IncomingCircuitKey:            packet.IncomingCircuitKey,