sheep: fix error in sheepdog cluster recovery

Sheepdog failed to recover object when we running it on 5 servers cluster with
about 20G data by erasure-code mode.

The reason is in default_create_and_write(): it rename() obj to data-directory
and then set xattr of ec-index for it, this will leave a time-window for another
process to read the data-object but can't get xattr of ec-index. Then the
process will report get-xattr fail and remove the disk as it think it's an
io-error event.

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