Summary:
**Context/Summary:**
https://github.com/facebook/rocksdb/pull/12542 introduced a bug where wrong padded bytes used to generate file checksum if flush happens during padding. This PR fixed it along with an existing same bug for `perform_data_verification_=true`.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12598
Test Plan:
- New UT that failed before this fix (`db->VerifyFileChecksums: ...Corruption: ...file checksum mismatch`) and passes after
- Benchmark
```
TEST_TMPDIR=/dev/shm ./db_bench --benchmarks=fillseq[-X300] --num=100000 --block_align=1 --compression_type=none
```
Pre-PR:
fillseq [AVG 300 runs] : 421334 (± 4126) ops/sec; 46.6 (± 0.5) MB/sec
Post-PR: (no regression observed but a slight improvement)
fillseq [AVG 300 runs] : 425768 (± 4309) ops/sec; 47.1 (± 0.5) MB/sec
Reviewed By: ajkr, anand1976
Differential Revision: D56725688
Pulled By: hx235
fbshipit-source-id: c1a700a95def8c65c0a21e44f8c1966164925ad5
abd6751aba