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GH-40306: [C++] Fix a wrong total_bytes to generate StringType's test data in vector_hash_benchmark (#40307)



### Rationale for this change
Fix a wrong total_bytes to generate StringType's test data in vector_hash_benchmark

### What changes are included in this PR?
Use  `params.length*byte_width` as the length of `std::vector<uint8_t> uniques` array.

### Are these changes tested?

### Are there any user-facing changes?
No

* GitHub Issue: #40306

Authored-by: hugo.zhang <hugo.zhang@openpie.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>