multi: enforce routing.MinCLTVDelta=18 for invoices + chanupd

This commit clamps all user-chosen CLTVs in LND to be at least 18, which
is the new conservative value used in the sepc. This minimum is applied
uniformly to forwarding CLTV deltas (via channel updates) as well as
final CLTV deltas for new invoices.

lnrpc/invoicesrpc/addinvoice.go

@@ -17,6 +17,7 @@ import (
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/netann"
+	"github.com/lightningnetwork/lnd/routing"
 	"github.com/lightningnetwork/lnd/zpay32"
 )
 
@@ -228,6 +229,14 @@ func AddInvoice(ctx context.Context, cfg *AddInvoiceConfig,
 		return nil, nil, fmt.Errorf("CLTV delta of %v is too large, max "+
 			"accepted is: %v", invoice.CltvExpiry, math.MaxUint16)
 	case invoice.CltvExpiry != 0:
+		// Disallow user-chosen final CLTV deltas below the required
+		// minimum.
+		if invoice.CltvExpiry < routing.MinCLTVDelta {
+			return nil, nil, fmt.Errorf("CLTV delta of %v must be "+
+				"greater than minimum of %v",
+				routing.MinCLTVDelta, invoice.CltvExpiry)
+		}
+
 		options = append(options,
 			zpay32.CLTVExpiry(invoice.CltvExpiry))
 	default: