rgw: update swift subuser perm masks when authenticating

Fixes: #9918
Backport: firefly, giant
It seems that we weren't setting the swift perm mask correctly.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
Merge pull request #2867 from ceph/wip-9918

rgw: update swift subuser perm masks when authenticating

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>