rgw: fix authenticated users acl group check

Fixes: #6553
Backport: bobtail, cuttlefish, dumpling
Authenticated users group acl bit was not working correctly. Check to
test whether user is anonymous was wrong.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #718 from ceph/wip-6553

rgw: fix authenticated users acl group check

Reviewed-by: Sage Weil <sage@inktank.com>