sm: correct Cancel tx size

Units were mixed up. Correct and clarify the code to avoid another
overlook.

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

Model/statemachine.py

@@ -162,18 +162,10 @@ def _feerate_to_fee(self, feerate, tx_type, n_fb_inputs):
         """
         if tx_type not in ["cancel", "emergency", "unemergency"]:
             raise ValueError("Invalid tx_type")
-        # feerate in satoshis/vbyte, weights in WU == 4vbytes, so feerate*weight/4 gives satoshis
-        return round(
-            feerate
-            * (
-                int(
-                    CANCEL_TX_WEIGHT[self.n_stk][self.n_man]
-                    + n_fb_inputs * P2WPKH_INPUT_SIZE
-                )
-                / 4
-            ),
-            0,
-        )
+        # feerate is in satoshis/vbyte
+        cancel_tx_size_no_fb = (CANCEL_TX_WEIGHT[self.n_stk][self.n_man] + 3) // 4
+        cancel_tx_size = cancel_tx_size_no_fb + n_fb_inputs * P2WPKH_INPUT_SIZE
+        return cancel_tx_size * feerate
 
     def fee_reserve_per_vault(self, block_height):
         return self._feerate_to_fee(