ReplicatedPG::make_writeable: fill in ssc on clone

ssc should always be filled in on live ObjectContexts.

Fixes: #7835
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1555 from ceph/wip-7835

ReplicatedPG::make_writeable: fill in ssc on clone

Reviewed-by: Sage Weil <sage@inktank.com>