fix memory alignment with logical sector size
fix memory alignment with logical sector size

Summary:
we align the buffer with logical sector size and should not test it with page size, which is usually 4k.
Closes https://github.com/facebook/rocksdb/pull/2245

Differential Revision: D5001842

Pulled By: lightmark

fbshipit-source-id: a7135fcf6351c6db363e8908956b1e193a4a6291

vmx/rocksdb,vmx/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,bbiao/rocksdb,bbiao/rocksdb,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,vmx/rocksdb,bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,vmx/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,vmx/rocksdb,bbiao/rocksdb,vmx/rocksdb