db_bench: --soft_pending_compaction_bytes_limit should set options.soft_pending_compaction_bytes_limit
db_bench: --soft_pending_compaction_bytes_limit should set options.soft_pending_compaction_bytes_limit

Summary: Fix a bug that options.soft_pending_compaction_bytes_limit is not actually set with --soft_pending_compaction_bytes_limit

Test Plan: Run db_bench with this parameter and make sure the parameter is set correctly.

Reviewers: anthony, kradhakrishnan, yhchiang, IslamAbdelRahman, igor, rven

Reviewed By: rven

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D52125

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