ln/refactor: move on chain timeout check into claimable htlc

diff --git a/lightning/src/ln/channelmanager.rs b/lightning/src/ln/channelmanager.rs
index 26eb42d..cd1eb39 100644
--- a/lightning/src/ln/channelmanager.rs
+++ b/lightning/src/ln/channelmanager.rs
@@ -535,6 +535,14 @@ struct MppPart {
 	total_value_received: Option<u64>,
 }
 
+impl MppPart {
+	/// Returns a boolean indicating whether the HTLC has timed out on chain, accounting for a buffer
+	/// that gives us time to resolve it.
+	fn check_onchain_timeout(&self, height: u32) -> bool {
+		height >= self.cltv_expiry - HTLC_FAIL_BACK_BUFFER
+	}
+}
+
 impl PartialOrd for MppPart {
 	fn partial_cmp(&self, other: &MppPart) -> Option<cmp::Ordering> {
 		Some(self.cmp(other))
@@ -16214,14 +16222,15 @@ impl<
 		}
 
 		if let Some(height) = height_opt {
+			// If height is approaching the number of blocks we think it takes us to get our
+			// commitment transaction confirmed before the HTLC expires, plus the number of blocks
+			// we generally consider it to take to do a commitment update, just give up on it and
+			// fail the HTLC.
 			self.claimable_payments.lock().unwrap().claimable_payments.retain(
 				|payment_hash, payment| {
 					payment.htlcs.retain(|htlc| {
-						// If height is approaching the number of blocks we think it takes us to get
-						// our commitment transaction confirmed before the HTLC expires, plus the
-						// number of blocks we generally consider it to take to do a commitment update,
-						// just give up on it and fail the HTLC.
-						if height >= htlc.mpp_part.cltv_expiry - HTLC_FAIL_BACK_BUFFER {
+						let htlc_timed_out = htlc.mpp_part.check_onchain_timeout(height);
+						if htlc_timed_out {
 							let reason = LocalHTLCFailureReason::PaymentClaimBuffer;
 							timed_out_htlcs.push((
 								HTLCSource::PreviousHopData(htlc.mpp_part.prev_hop.clone()),
@@ -16234,10 +16243,8 @@ impl<
 									payment_hash: payment_hash.clone(),
 								},
 							));
-							false
-						} else {
-							true
 						}
+						!htlc_timed_out
 					});
 					!payment.htlcs.is_empty() // Only retain this entry if htlcs has at least one entry.
 				},