rgw: set meta object in extra flag when initializing it

As part of the fix for 8452 we moved the meta object initialization.
Missed moving the extra flag initialization that is needed. This breaks
setups where there's a separate extra pool (needed in ec backends).

Reported-by: Sylvain Munaut <s.munaut@whatever-company.com>
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 703d0eb9bffdf6c553b659728a3a07f068fb15ee)