lnwallet: fix validateFeeRate

The validateFeeRate function uses the availableBalance function to get
the current spendable balance of a channel, adds the old fee and then
ensures that the new fee is not larger than the amount we have available
to spend. This commit also removes the local reserve check in the
validateFeeRate function since the balance returned from
availableBalance already takes the local reserve into acccount.

lnwallet/channel.go

@@ -6821,7 +6821,10 @@ func (lc *LightningChannel) validateFeeRate(feePerKw chainfee.SatPerKWeight) err
 	// be above our reserve balance. Otherwise, we'll reject the fee
 	// update.
 	availableBalance, txWeight := lc.availableBalance()
-	oldFee := lnwire.NewMSatFromSatoshis(lc.localCommitChain.tip().fee)
+
+	oldFee := lnwire.NewMSatFromSatoshis(
+		lc.localCommitChain.tip().feePerKw.FeeForWeight(txWeight),
+	)
 
 	// Our base balance is the total amount of satoshis we can commit
 	// towards fees before factoring in the channel reserve.
@@ -6843,17 +6846,6 @@ func (lc *LightningChannel) validateFeeRate(feePerKw chainfee.SatPerKWeight) err
 			newFee, baseBalance)
 	}
 
-	// If this new balance is below our reserve, then we can't accommodate
-	// the fee change, so we'll reject it.
-	balanceAfterFee := baseBalance - newFee
-	if balanceAfterFee.ToSatoshis() < lc.channelState.LocalChanCfg.ChanReserve {
-		return fmt.Errorf("cannot apply fee_update=%v sat/kw, "+
-			"new balance=%v would dip below channel reserve=%v",
-			int64(feePerKw),
-			balanceAfterFee.ToSatoshis(),
-			lc.channelState.LocalChanCfg.ChanReserve)
-	}
-
 	// TODO(halseth): should fail if fee update is unreasonable,
 	// as specified in BOLT#2.
 	//  * COMMENT(roasbeef): can cross-check with our ideal fee rate