Fix the wrong address for PREFETCH in DynamicBloom::Prefetch

Summary:
- Change data_[b] to data_[b / 8] in DynamicBloom::Prefetch, as b means the b-th bit in data_ and data_[b / 8] is the proper byte in data_.
Closes https://github.com/facebook/rocksdb/pull/1935

Differential Revision: D4628696

Pulled By: siying

fbshipit-source-id: bc5a0c6

util/dynamic_bloom.h

@@ -128,7 +128,7 @@ inline bool DynamicBloom::MayContain(const Slice& key) const {
 inline void DynamicBloom::Prefetch(uint32_t h) {
   if (kNumBlocks != 0) {
     uint32_t b = ((h >> 11 | (h << 21)) % kNumBlocks) * (CACHE_LINE_SIZE * 8);
-    PREFETCH(&(data_[b]), 0, 3);
+    PREFETCH(&(data_[b / 8]), 0, 3);
   }
 }