avoid ftruncate twice in buffered io
avoid ftruncate twice in buffered io

Summary:
in buffered io, the filesize_ is the real size.
Closes https://github.com/facebook/rocksdb/pull/1991

Differential Revision: D4711433

Pulled By: lightmark

fbshipit-source-id: ad604b9

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