rgw: fix reading bucket policy in RGWBucket::get_policy()

Fixes: 6940
Backport: dumpling, emperor

We changed the way we keep the bucket policy, and we shouldn't try to
access the bucket object directly. This had changed when we added the
bucket instance object around dumpling.

Reported-by: Gao, Wei M <wei.m.gao@intel.com>
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>