Return correct usable_size for BlockContents (#4246)

Summary:
If jemalloc is disabled or the API is incorrectly referenced (jemalloc api on windows have a prefix je_) memory usage is incorrectly reported for all block sizes. This is because sizeof(char) is always 1. sizeof() is calculated at compile time and *(char*) is char. The patch uses the size of the slice to fix that.
Fixes #4245
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4246

Differential Revision: D9233958

Pulled By: maysamyabandeh

fbshipit-source-id: 9646933b24504e2814c7379f06a31148829c6b4e

table/format.h

@@ -212,7 +212,7 @@ struct BlockContents {
 #ifdef ROCKSDB_MALLOC_USABLE_SIZE
       return malloc_usable_size(allocation.get());
 #else
-      return sizeof(*allocation.get());
+      return data.size();
 #endif  // ROCKSDB_MALLOC_USABLE_SIZE
     } else {
       return 0;  // no extra memory is occupied by the data