rgw: S3 Put Bucket Policy should return 204 on success

rgw: S3 Put Bucket Policy should return 204 on success

Currently RGW returns a 200 on a successful PUT on a bucket policy but
the S3 api extects a 204, which makes sense as it's a success without
any contect returned:

   https://docs.aws.amazon.com/AmazonS3/latest/API/RESTBucketPUTpolicy.html

The sample response in the example above is a 204.

This patch checks the op_ret in `RGWPutBucketPolicy::send_response()`
and on a success we turn it to a 204 (or STATUS_NO_CONTENT).

Fixes: https://tracker.ceph.com/issues/45467
Signed-off-by: Matthew Oliver <moliver@suse.com>