Use parent blockhash to seed EpochRewardsHasher (solana-labs#34744)

* Use parent blockhash to seed rewards-partition hasher

* Make blockhash word more consistent

runtime/src/bank.rs

@@ -2355,9 +2355,13 @@ impl Bank {
             .unwrap_or_default();
 
         let num_partitions = self.get_reward_distribution_num_blocks(&stake_rewards.stake_rewards);
+        let parent_blockhash = self
+            .parent()
+            .expect("Partitioned rewards calculation must still have access to parent Bank.")
+            .last_blockhash();
         let stake_rewards_by_partition = hash_rewards_into_partitions(
             std::mem::take(&mut stake_rewards.stake_rewards),
-            &self.parent_hash(),
+            &parent_blockhash,
             num_partitions as usize,
         );
 

runtime/src/epoch_rewards_hasher.rs

@@ -42,10 +42,10 @@ fn hash_to_partition(hash: u64, partitions: usize) -> usize {
 
 pub(crate) fn hash_rewards_into_partitions(
     stake_rewards: StakeRewards,
-    parent_block_hash: &Hash,
+    parent_blockhash: &Hash,
     num_partitions: usize,
 ) -> Vec<StakeRewards> {
-    let hasher = EpochRewardsHasher::new(num_partitions, parent_block_hash);
+    let hasher = EpochRewardsHasher::new(num_partitions, parent_blockhash);
     let mut result = vec![vec![]; num_partitions];
 
     for reward in stake_rewards {