Use regular hmac comparison in onion failure processing

Because this method is used by the sender of the payment after the htlc
has been resolved, there is no information that can leak to downstream
nodes. Revert to a regular comparison for better performance and easier
fuzzing.

Author: joostjager

lightning/src/ln/onion_utils.rs

@@ -1160,7 +1160,7 @@ where
 		let mut hmac = HmacEngine::<Sha256>::new(&um);
 		hmac.input(&encrypted_packet.data[32..]);
 
-		if !fixed_time_eq(&Hmac::from_engine(hmac).to_byte_array(), &encrypted_packet.data[..32]) {
+		if &Hmac::from_engine(hmac).to_byte_array() != &encrypted_packet.data[..32] {
 			continue;
 		}