db_bench_tool: fix buffer size
db_bench_tool: fix buffer size

Summary:
Found by gcc warning:

x86_64-pc-linux-gnu-g++ --version
x86_64-pc-linux-gnu-g++ (GCC) 7.1.1 20170710

tools/db_bench_tool.cc: In member function 'void rocksdb::Benchmark::RandomWithVerify(rocksdb::ThreadState*)':
tools/db_bench_tool.cc:4430:8: error: '%lu' directive output may be truncated writing between 1 and 19 bytes into a region of size between 0 and 66 [-Werror=format-truncation=]
   void RandomWithVerify(ThreadState* thread) {
        ^~~~~~~~~~~~~~~~
tools/db_bench_tool.cc:4430:8: note: directive argument in the range [0, 9223372036854775807]
tools/db_bench_tool.cc:4492:13: note: 'snprintf' output between 37 and 128 bytes into a destination of size 100
     snprintf(msg, sizeof(msg),
     ~~~~~~~~^~~~~~~~~~~~~~~~~~
              "( get:%" PRIu64 " put:%" PRIu64 " del:%" PRIu64 " total:%" \
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
              PRIu64 " found:%" PRIu64 ")",
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
              gets_done, puts_done, deletes_done, readwrites_, found);
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
Makefile:1707: recipe for target 'tools/db_bench_tool.o' failed
Closes https://github.com/facebook/rocksdb/pull/2558

Differential Revision: D5398703

Pulled By: siying

fbshipit-source-id: 6ffa552bbd8b59cfc2c36289f86ff9b9acca8ca6

SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb