[Java] Fixed a bug in Java DB Benchmark where random reads does not consider full key range.

Summary: Fixed a bug in Java DB Benchmark where random reads does not consider full key range.

Test Plan:
make rocksdbjava
make jdb_bench
cd java
jdb_bench.sh --db=/tmp/rocksdb-test --benchmarks=fillseq --use_existing_db=false --num=100000
jdb_bench.sh --db=/tmp/rocksdb-test --benchmarks=readrandom --use_existing_db=true --num=100000 --reads=1000000

Reviewers: haobo, sdong

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18693

java/org/rocksdb/benchmark/DbBenchmark.java

@@ -395,7 +395,7 @@ public class DbBenchmark {
       byte[] key = new byte[keySize_];
       byte[] value = new byte[valueSize_];
       for (long i = 0; i < numEntries_; i++) {
-        getRandomKey(key, numEntries_);
+        getRandomKey(key, keyRange_);
         int len = db_.get(key, value);
         if (len != RocksDB.NOT_FOUND) {
           stats_.found_++;