db_bench: let -benchmark=compact respect -subcompactions (#11077)

db_bench: let -benchmark=compact respect -subcompactions (#11077)

Summary:
When running `-benchmarks=compact`, `-subcompactions` does not take effect.

`-subcompactions` option comment says it is for L0-L1 compactions, it is natural to extend it to CompactionRangeOptions.max_subcompactions.

This PR set CompactionRangeOptions.max_subcompactions = FLAGS_subcompactions

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11077

Reviewed By: akankshamahajan15

Differential Revision: D42506251

Pulled By: ajkr

fbshipit-source-id: f77c9a99d32ff7af59f3c452c9e16aaeb0360304