sheep/kv: initialize return code in onode_populate_data()

When we upload a small inlined-object, the function onode_populate_data()
will only return the variable 'ret' which has not been initialized.
So, we should set 'ret' to SD_RES_SUCCESS firstly.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>