link+lnwallet: move bandwidth channel reserve validation into channel

Since we want to handle the edge case where paying the HTLC fee would
take the initiator below the reserve, we move the subtraction of the
reserve into availableBalance where this calculation will be performed.

htlcswitch/link.go

@@ -2127,26 +2127,21 @@ func (l *channelLink) ChanID() lnwire.ChannelID {
 //
 // NOTE: Part of the ChannelLink interface.
 func (l *channelLink) Bandwidth() lnwire.MilliSatoshi {
+	// Get the balance available on the channel for new HTLCs. This takes
+	// the channel reserve into account so HTLCs up to this value won't
+	// violate it.
 	channelBandwidth := l.channel.AvailableBalance()
-	overflowBandwidth := l.overflowQueue.TotalHtlcAmount()
 
 	// To compute the total bandwidth, we'll take the current available
 	// bandwidth, then subtract the overflow bandwidth as we'll eventually
 	// also need to evaluate those HTLC's once space on the commitment
 	// transaction is free.
-	linkBandwidth := channelBandwidth - overflowBandwidth
-
-	// If the channel reserve is greater than the total available balance
-	// of the link, just return 0.
-	reserve := lnwire.NewMSatFromSatoshis(l.channel.LocalChanReserve())
-	if linkBandwidth < reserve {
+	overflowBandwidth := l.overflowQueue.TotalHtlcAmount()
+	if channelBandwidth < overflowBandwidth {
 		return 0
 	}
 
-	// Else the amount that is available to flow through the link at this
-	// point is the available balance minus the reserve amount we are
-	// required to keep as collateral.
-	return linkBandwidth - reserve
+	return channelBandwidth - overflowBandwidth
 }
 
 // AttachMailBox updates the current mailbox used by this link, and hooks up