rgw: read attributes when reading bucket entry point

Fixes: #5691

We need to also read the attributes, as bucket might be a legacy
bucket and might have all bucket instance info in that object.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
Tested-by: Faidon Liambotis <faidon@wikimedia.org>