Set ChannelUpdate htlc_maximum_msat using the peer's value

Use the `counterparty_max_htlc_value_in_flight_msat` value, and not the
`holder_max_htlc_value_in_flight_msat` value when creating the
`htlc_maximum_msat` value for `ChannelUpdate` messages.

BOLT 7 specifies that the field MUST be less than or equal to
`max_htlc_value_in_flight_msat` received from the peer, which we
currently are not guaranteed to adhere to by using the holder value.

Author: ViktorTigerstrom

lightning/src/ln/channel.rs

@@ -4357,7 +4357,7 @@ impl<Signer: Sign> Channel<Signer> {
 			// channel might have been used to route very small values (either by honest users or as DoS).
 			self.channel_value_satoshis * 1000 * 9 / 10,
 
-			self.holder_max_htlc_value_in_flight_msat
+			self.counterparty_max_htlc_value_in_flight_msat
 		);
 	}