Fix db_bench duration for multireadrandom benchmark (#7817)

Summary:
The multireadrandom benchmark, when run for a specific number of reads (--reads argument), should base the duration on the actual number of keys read rather than number of batches.

Tests:
Run db_bench multireadrandom benchmark

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7817

Reviewed By: zhichao-cao

Differential Revision: D25717230

Pulled By: anand1976

fbshipit-source-id: 13f4d8162268cf9a34918655e60302d0aba3864b

tools/db_bench_tool.cc

@@ -5220,7 +5220,7 @@ class Benchmark {
     }
 
     Duration duration(FLAGS_duration, reads_);
-    while (!duration.Done(1)) {
+    while (!duration.Done(entries_per_batch_)) {
       DB* db = SelectDB(thread);
       if (FLAGS_multiread_stride) {
         int64_t key = GetRandomKey(&thread->rand);