fix compile error on 32-bits

Signed-off-by: xinxin shu <xinxin.shu@intel.com>
> on 32-bits system , if file size is larger than 2GB, which means large file support has been enabled,
> then off_t equals to off64_t

I think this is controlled via 

 -D_FILE_OFFSET_BITS=64

in Makefile-env.am for ceph, but does rocksdb do the same?  I don't think 
defining that (or not) is directly related to whether the files get that 
big unless the rocksdb devs paid particular attention to that...