rgw/reshard: use defined variable

rgw/reshard: use defined variable

use the defined reference for more concise code, this silences the
warning like:
```
../src/rgw/rgw_reshard.cc:530:15: warning: unused variable ‘bucket’ [-Wunused-variable]
  530 |   rgw_bucket& bucket = bucket_info.bucket;
      |               ^~~~~~
```

also move `ret` close to where it is used for the first time.

Signed-off-by: Kefu Chai <kchai@redhat.com>