rgw: fix copy object err when bucket with specified placement rule
one can reproduce with steps:
1. create a src bucket with a specified placement rule, which has different data pool from the default placement rule.
2. create a dst bucket with same placement rule as the src bucket
3. upload a large object test_obj using multipart to src bucket
4. do s3cmd cp s3://src/test_obj s3://dst/test_obj
then will see the result:
WARNING: Key not found s3://src/test_obj