Summary:
This is to avoid run time error. Fail the db_bench immediately if cuckoo table is used but mmap_read is not specified.
Closes https://github.com/facebook/rocksdb/pull/3420

Differential Revision: D6838284

Pulled By: siying

fbshipit-source-id: 20893fa28d40fadc31e4ff154bed02f5a1bad341