Use checked math in frame-balances named_reserve (#7365)

This PR modifies `named_reserve()` in frame-balances to use checked math
instead of defensive saturating math.

The use of saturating math relies on the assumption that the sum of the
values will always fit in `u128::MAX`. However, there is nothing
preventing the implementing pallet from passing a larger value which
overflows. This can happen if the implementing pallet does not validate
user input and instead relies on `named_reserve()` to return an error
(this saves an additional read)

This is not a security concern, as the method will subsequently return
an error thanks to `<Self as ReservableCurrency<_>>::reserve(who,
value)?;`. However, the `defensive_saturating_add` will panic in
`--all-features`, creating false positive crashes in fuzzing operations.

---------

Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>

prdoc/pr_7365.prdoc

@@ -0,0 +1,12 @@
+title: Use checked math in frame-balances named_reserve
+doc:
+- audience: Runtime Dev
+  description: |-
+    This PR modifies `named_reserve()` in frame-balances to use checked math instead of defensive saturating math.
+
+    The use of saturating math relies on the assumption that the value will always fit in `u128::MAX`. However, there is nothing preventing the implementing pallet from passing a larger value which overflows. This can happen if the implementing pallet does not validate user input and instead relies on `named_reserve()` to return an error (this saves an additional read)
+
+    This is not a security concern, as the method will subsequently return an error thanks to `<Self as ReservableCurrency<_>>::reserve(who, value)?;`. However, the `defensive_saturating_add` will panic in `--all-features`, creating false positive crashes in fuzzing operations.
+crates:
+- name: pallet-balances
+  bump: patch

substrate/frame/balances/src/impl_currency.rs

@@ -674,8 +674,10 @@ where
 		Reserves::<T, I>::try_mutate(who, |reserves| -> DispatchResult {
 			match reserves.binary_search_by_key(id, |data| data.id) {
 				Ok(index) => {
-					// this add can't overflow but just to be defensive.
-					reserves[index].amount = reserves[index].amount.defensive_saturating_add(value);
+					reserves[index].amount = reserves[index]
+						.amount
+						.checked_add(&value)
+						.ok_or(ArithmeticError::Overflow)?;
 				},
 				Err(index) => {
 					reserves