Check htlc_maximum_msat and switch limit

As our outbound channel liquidity might already be limited by the
`htlc_maximum_msat` which is considered during route finding, it doesn't
make sense to further restrict the liquidity for payment probes as
sending probes wouldn't necessarily cut into the liquidity accounted for
in `next_outbound_htlc_limit_msat`.

We therefore detect if we're limited by `htlc_maximum_msat`, and if so,
we choose to use our current outbound capacity as the basis for our
calculation.

Author: tnull

lightning/src/ln/channelmanager.rs

@@ -3537,7 +3537,15 @@ where
 					let used_liquidity =
 						used_liquidity_map.entry(first_path_hop.short_channel_id).or_insert(0);
 
-					if first_hop.next_outbound_htlc_limit_msat
+					// If we're already limited through `htlc_maximum_msat`, we use the available outbound capacity
+					// to calculate the liquidity limit.
+					let relevant_limit = if Some(first_hop.next_outbound_htlc_limit_msat) == first_hop.counterparty.outbound_htlc_maximum_msat {
+						first_hop.outbound_capacity_msat
+					} else {
+						first_hop.next_outbound_htlc_limit_msat
+					};
+
+					if relevant_limit
 						< (*used_liquidity + path_value)
 							* self.default_configuration.preflight_probing_liquidity_limit_multiplier
 					{