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/mod.rs

@@ -564,6 +564,12 @@ pub mod pallet {
 	#[pallet::getter(fn force_era)]
 	pub type ForceEra<T> = StorageValue<_, Forcing, ValueQuery>;
 
+	/// Maximum staked rewards, i.e. the percentage of the era inflation that
+	/// is used for stake rewards.
+	/// See [Era payout](./index.html#era-payout).
+	#[pallet::storage]
+	pub type MaxStakedRewards<T> = StorageValue<_, Percent, OptionQuery>;
+
 	/// The percentage of the slash that is distributed to reporters.
 	///
 	/// The rest of the slashed value is handled by the `Slash`.
@@ -1717,6 +1723,7 @@ pub mod pallet {
 			max_validator_count: ConfigOp<u32>,
 			chill_threshold: ConfigOp<Percent>,
 			min_commission: ConfigOp<Perbill>,
+			max_staked_rewards: ConfigOp<Percent>,
 		) -> DispatchResult {
 			ensure_root(origin)?;
 
@@ -1736,6 +1743,7 @@ pub mod pallet {
 			config_op_exp!(MaxValidatorsCount<T>, max_validator_count);
 			config_op_exp!(ChillThreshold<T>, chill_threshold);
 			config_op_exp!(MinCommission<T>, min_commission);
+			config_op_exp!(MaxStakedRewards<T>, max_staked_rewards);
 			Ok(())
 		}
 		/// Declare a `controller` to stop participating as either a validator or nominator.