table/block.h: change memset

Summary:
In gcc-7 the following is an error identified by -Werror=class-memaccess

In file included from ./table/get_context.h:14:0,
                 from db/version_set.cc:43:
./table/block.h: In constructor ‘rocksdb::BlockReadAmpBitmap::BlockReadAmpBitmap(size_t, size_t, rocksdb::Statistics*)’:
./table/block.h:73:53: error: ‘void* memset(void*, int, size_t)’ clearing an object of type ‘struct std::atomic<unsigned int>’ with no trivial copy-assignment; use value-initialization instead [-Werror=class-memaccess]
     memset(bitmap_, 0, bitmap_size * kBytesPersEntry);
                                                     ^
In file included from ./db/version_set.h:23:0,
                 from db/version_set.cc:12:
/toolchain/include/c++/8.0.0/atomic:684:12: note: ‘struct std::atomic<unsigned int>’ declared here
     struct atomic<unsigned int> : __atomic_base<unsigned int>
            ^~~~~~~~~~~~~~~~~~~~

As a solution the default initializer can be applied in list context.

Signed-off-by: Daniel Black <daniel.black@au.ibm.com>
Closes https://github.com/facebook/rocksdb/pull/2561

Differential Revision: D5398714

Pulled By: siying

fbshipit-source-id: d883fb88ec7535eee60d551038fe91f14488be36

table/block.h

@@ -67,8 +67,7 @@ class BlockReadAmpBitmap {
     size_t bitmap_size = (num_bits_needed - 1) / kBitsPerEntry + 1;
 
     // Create bitmap and set all the bits to 0
-    bitmap_ = new std::atomic<uint32_t>[bitmap_size];
-    memset(bitmap_, 0, bitmap_size * kBytesPersEntry);
+    bitmap_ = new std::atomic<uint32_t>[bitmap_size]();
 
     RecordTick(GetStatistics(), READ_AMP_TOTAL_READ_BYTES, block_size);
   }