sheep: make stat_data_objs a generic function

and rename it as sd_inode_stat()

Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep/nfs: implement nfs3_fsstat operation

Now we support 'df' command to get file system stats.

Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>