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>
(cherry picked from commit bebbd6cb7b71697b34b8f27652cabdc40c97a33b)