ReplicatedPG::agent_work: skip hitset objects before getting object context

Otherwise, we might read the attr on a hitset object we are in the
process of deleting.

Fixes: #8086
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1674 from ceph/wip-8086

ReplicatedPG::agent_work: skip hitset objects before getting object cont...

Reviewed-by: Sage Weil <sage@inktank.com>