Fix Mac build break in casting (#4722)

Summary:
Mac build is failing with the below error:
```
$ make db_bench -j8
...
...
tools/db_bench_tool.cc:4583:25: error: no matching function for call to 'max'
              (uint64_t)std::max(0l, seek_pos - FLAGS_max_scan_distance),
                        ^~~~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/algorithm:2717:1: note: candidate template ignored: deduced conflicting types for parameter '_Tp' ('long' vs. 'long long')
max(const _Tp& __a, const _Tp& __b)
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/algorithm:2727:1: note: candidate template ignored: could not match 'initializer_list<type-parameter-0-0>' against 'long'
max(initializer_list<_Tp> __t, _Compare __comp)
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/algorithm:2709:1: note: candidate function template not viable: requires 3 arguments, but 2 were provided
max(const _Tp& __a, const _Tp& __b, _Compare __comp)
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/algorithm:2735:1: note: candidate function template not viable: requires single argument '__t', but 2 arguments were provided
max(initializer_list<_Tp> __t)
^
1 error generated.
make: *** [tools/db_bench_tool.o] Error 1
```

My compiler version:
Mac OS X Mojave
```
$ clang++ --version
Apple LLVM version 10.0.0 (clang-1000.11.45.5)
Target: x86_64-apple-darwin18.2.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4722

Differential Revision: D13220196

Pulled By: sagar0

fbshipit-source-id: 01e5e928288a5613027c83a26ad8aedf04438b14

tools/db_bench_tool.cc

@@ -4580,7 +4580,7 @@ void VerifyDBFromDB(std::string& truth_db_name) {
       if (FLAGS_max_scan_distance != 0) {
         if (FLAGS_reverse_iterator) {
           GenerateKeyFromInt(
-              (uint64_t)std::max(0l, seek_pos - FLAGS_max_scan_distance),
+              (uint64_t)std::max((int64_t)0, seek_pos - FLAGS_max_scan_distance),
               FLAGS_num, &lower_bound);
           options.iterate_lower_bound = &lower_bound;
         } else {