Implements a % cap on staking rewards from era inflation (#1660)

This PR implements an (optional) cap of the era inflation that is
allocated to staking rewards. The remaining is minted directly into the
[`RewardRemainder`](https://github.com/paritytech/polkadot-sdk/blob/fb0fd3e62445eb2dee2b2456a0c8574d1ecdcc73/substrate/frame/staking/src/pallet/mod.rs#L160)
account, which is the treasury pot account in Polkadot and Kusama.

The staking pallet now has a percent storage item, `MaxStakersRewards`,
which defines the max percentage of the era inflation that should be
allocated to staking rewards. The remaining era inflation (i.e.
`remaining = max_era_payout - staking_payout.min(staking_payout *
MaxStakersRewards))` is minted directly into the treasury.

The `MaxStakersRewards` can be set by a privileged origin through the
`set_staking_configs` extrinsic.

**To finish**
- [x] run benchmarks for westend-runtime

Replaces https://github.com/paritytech/polkadot-sdk/pull/1483
Closes https://github.com/paritytech/polkadot-sdk/issues/403

---------

Co-authored-by: command-bot <>

substrate/frame/staking/src/pallet/impls.rs

@@ -36,7 +36,7 @@ use frame_system::{pallet_prelude::BlockNumberFor, RawOrigin};
 use pallet_session::historical;
 use sp_runtime::{
 	traits::{Bounded, Convert, One, SaturatedConversion, Saturating, StaticLookup, Zero},
-	Perbill,
+	Perbill, Percent,
 };
 use sp_staking::{
 	currency_to_vote::CurrencyToVote,
@@ -507,9 +507,18 @@ impl<T: Config> Pallet<T> {
 				.saturated_into::<u64>();
 			let staked = Self::eras_total_stake(&active_era.index);
 			let issuance = T::Currency::total_issuance();
+
 			let (validator_payout, remainder) =
 				T::EraPayout::era_payout(staked, issuance, era_duration);
 
+			let total_payout = validator_payout.saturating_add(remainder);
+			let max_staked_rewards =
+				MaxStakedRewards::<T>::get().unwrap_or(Percent::from_percent(100));
+
+			// apply cap to validators payout and add difference to remainder.
+			let validator_payout = validator_payout.min(max_staked_rewards * total_payout);
+			let remainder = total_payout.saturating_sub(validator_payout);
+
 			Self::deposit_event(Event::<T>::EraPaid {
 				era_index: active_era.index,
 				validator_payout,