Use onion amount amt_to_forward for MPP set calculation

If routing nodes take less fees and pay the final node more than
`amt_to_forward`, the receiver may see that `total_msat` has been met
before all of the sender's intended HTLCs have arrived. The receiver
may then prematurely claim the payment and release the payment hash,
allowing routing nodes to claim the remaining HTLCs. Using the onion
value `amt_to_forward` to determine when `total_msat` has been met
allows the sender to control the set total.

Author: alecchendev

lightning/src/ln/channelmanager.rs

@@ -2108,7 +2108,7 @@ where
 			payment_hash,
 			incoming_shared_secret: shared_secret,
 			incoming_amt_msat: Some(amt_msat),
-			outgoing_amt_msat: amt_msat,
+			outgoing_amt_msat: hop_data.amt_to_forward,
 			outgoing_cltv_value: hop_data.outgoing_cltv_value,
 		})
 	}