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>
Merge pull request #1915 from ceph/wip-8452-2

rgw: set meta object in extra flag when initializing it

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>