Page size isn't always 4k on linux

Summary:
Some places autodetected. These are the two places that didn't.

closes #1498

Still unsure if the following instances of 4 * 1024 need fixing in:
util/io_posix.h
include/rocksdb/table.h (appears to be blocksize and different)
utilities/persistent_cache/block_cache_tier.cc
utilities/persistent_cache/persistent_cache_test.h
include/rocksdb/env.h
util/env_posix.cc
db/column_family.cc
Closes https://github.com/facebook/rocksdb/pull/1499

Differential Revision: D4593640

Pulled By: yiwu-arbug

fbshipit-source-id: efc48de

util/env_test.cc

@@ -48,6 +48,12 @@
 #include "util/testharness.h"
 #include "util/testutil.h"
 
+#ifdef OS_LINUX
+static const size_t kPageSize = sysconf(_SC_PAGESIZE);
+#else
+static const size_t kPageSize = 4 * 1024;
+#endif
+
 namespace rocksdb {
 
 static const int kDelayMicros = 100000;
@@ -67,12 +73,12 @@ struct Deleter {
 std::unique_ptr<char, Deleter> NewAligned(const size_t size, const char ch) {
   char* ptr = nullptr;
 #ifdef OS_WIN
-  if (!(ptr = reinterpret_cast<char*>(_aligned_malloc(size, 4 * 1024)))) {
+  if (!(ptr = reinterpret_cast<char*>(_aligned_malloc(size, kPageSize)))) {
     return std::unique_ptr<char, Deleter>(nullptr, Deleter(_aligned_free));
   }
   std::unique_ptr<char, Deleter> uptr(ptr, Deleter(_aligned_free));
 #else
-  if (posix_memalign(reinterpret_cast<void**>(&ptr), 4 * 1024, size) != 0) {
+  if (posix_memalign(reinterpret_cast<void**>(&ptr), kPageSize, size) != 0) {
     return std::unique_ptr<char, Deleter>(nullptr, Deleter(free));
   }
   std::unique_ptr<char, Deleter> uptr(ptr, Deleter(free));