elements: Fix up feerate upper bound when updating the feerate

This was still using the non-elements estimation only and did not consider the
elements overhead.

channeld/full_channel.c

@@ -879,7 +879,28 @@ u32 approx_max_feerate(const struct channel *channel)
 	/* Assume none are trimmed; this gives lower bound on feerate. */
 	num = tal_count(committed) + tal_count(adding) - tal_count(removing);
 
+	/* BOLT #3:
+	 *
+	 * commitment_transaction: 125 + 43 * num-htlc-outputs bytes
+	 *	- version: 4 bytes
+	 *	- witness_header <---- part of the witness data
+	 *	- count_tx_in: 1 byte
+	 *	- tx_in: 41 bytes
+	 *		funding_input
+	 *	- count_tx_out: 1 byte
+	 *	- tx_out: 74 + 43 * num-htlc-outputs bytes
+	 *		output_paying_to_remote,
+	 *		output_paying_to_local,
+	 *		....htlc_output's...
+	 *	- lock_time: 4 bytes
+	 */
+	/* Those 74 bytes static output are effectively 2 outputs, one
+	 * `output_paying_to_local` and one `output_paying_to_remote`. So when
+	 * adding the elements overhead we need to add 2 + num htlcs
+	 * outputs. */
+
 	weight = 724 + 172 * num;
+	weight = elements_add_overhead(weight, 1, num + 2);
 
 	/* We should never go below reserve. */
 	if (!amount_sat_sub(&avail,