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>
(cherry picked from commit 561e7b0b287e65e90b80699e45a52ae44e94684f)