fix compile error on 32-bits

Signed-off-by: xinxin shu <xinxin.shu@intel.com>
i agree with that large file support is controlled via -D_FILE_OFFSET_BITS=64,  i  searched rocksdb code and didn't find any sign about _FILE_OFFSET_BITS,  i think rocksdb devs did not pay particular attention to that , so all files in rocksdb are not larger than 2GB , 64-bit type offset is not necessary .  even if rocksdb compile with D_FILE_OFFSET_BITS=64 , files in rocksdb can be beyond 2GB, but off_t equals to off64_t, so i think we do not have any problem