Fix an error in db_bench with gcc 4.8 (#6537)

Summary:
I start to see following failures:

tools/db_bench_tool.cc: In constructor ‘rocksdb::NormalDistribution::NormalDistribution(unsigned int, unsigned int)’:
tools/db_bench_tool.cc:1528:58: error: declaration of ‘max’ shadows a member of 'this' [-Werror=shadow]
   NormalDistribution(unsigned int min, unsigned int max) :
                                                          ^
tools/db_bench_tool.cc:1528:58: error: declaration of ‘min’ shadows a member of 'this' [-Werror=shadow]
tools/db_bench_tool.cc: In constructor ‘rocksdb::UniformDistribution::UniformDistribution(unsigned int, unsigned int)’:
tools/db_bench_tool.cc:1546:59: error: declaration of ‘max’ shadows a member of 'this' [-Werror=shadow]
   UniformDistribution(unsigned int min, unsigned int max) :
                                                           ^
tools/db_bench_tool.cc:1546:59: error: declaration of ‘min’ shadows a member of 'this' [-Werror=shadow]

when I build from GCC 4.8. Rename those variables to fix the problem.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6537

Test Plan: make all with the compiler that used to show the failure.

Differential Revision: D20448741

fbshipit-source-id: 18bcf012dbe020f22f79038a9b08f447befa2574

tools/db_bench_tool.cc

@@ -1484,9 +1484,8 @@ static enum DistributionType StringToDistributionType(const char* ctype) {
 
 class BaseDistribution {
  public:
-  BaseDistribution(unsigned int min, unsigned int max) :
+  BaseDistribution(unsigned int _min, unsigned int _max)
-    min_value_size_(min),
+      : min_value_size_(_min), max_value_size_(_max) {}
-    max_value_size_(max) {}
   virtual ~BaseDistribution() {}
 
   unsigned int Generate() {
@@ -1525,12 +1524,14 @@ class FixedDistribution : public BaseDistribution
 class NormalDistribution
     : public BaseDistribution, public std::normal_distribution<double> {
  public:
-  NormalDistribution(unsigned int min, unsigned int max) :
+  NormalDistribution(unsigned int _min, unsigned int _max)
-    BaseDistribution(min, max),
+      : BaseDistribution(_min, _max),
         // 99.7% values within the range [min, max].
-    std::normal_distribution<double>((double)(min + max) / 2.0 /*mean*/,
+        std::normal_distribution<double>(
-                                     (double)(max - min) / 6.0 /*stddev*/),
+            (double)(_min + _max) / 2.0 /*mean*/,
+            (double)(_max - _min) / 6.0 /*stddev*/),
         gen_(rd_()) {}
+
  private:
   virtual unsigned int Get() override {
     return static_cast<unsigned int>((*this)(gen_));
@@ -1543,10 +1544,11 @@ class UniformDistribution
     : public BaseDistribution,
       public std::uniform_int_distribution<unsigned int> {
  public:
-  UniformDistribution(unsigned int min, unsigned int max) :
+  UniformDistribution(unsigned int _min, unsigned int _max)
-    BaseDistribution(min, max),
+      : BaseDistribution(_min, _max),
-    std::uniform_int_distribution<unsigned int>(min, max),
+        std::uniform_int_distribution<unsigned int>(_min, _max),
         gen_(rd_()) {}
+
  private:
   virtual unsigned int Get() override {
     return (*this)(gen_);