rgw: initialize RGWUserAdminOpState::system_specified

Fixes: #6829
Backport: dumpling, emperor
We didn't init this member variable, which might cause that when
modifying user info that has this flag set the 'system' flag might
inadvertently reset.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #862 from ceph/wip-6829

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>