set readahead buffer size from roundup(user_size) + 4k to roundup(use…
set readahead buffer size from roundup(user_size) + 4k to roundup(use…

Summary:
Users usually set readahead buffer to a multiple of 4k, more than that, usually a multiple of blocks.
So previously we set real buffer size 512 * n + 4k, which may introduce an additional block reading.
Closes https://github.com/facebook/rocksdb/pull/2138

Differential Revision: D4871504

Pulled By: lightmark

fbshipit-source-id: b070faa51d92e976e8e8468c00692699e585e243

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