ReplicatedPG::find_object_context: check obs.exists on clone obc before checking snaps

Fixes: #7858
Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG::_scrub: account for clone_overlap on each clone

Otherwise, we end up subtracting off clone_overlap for evicted clones
whose sizes we did not add in.

FIxes: #7942
Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: use get_clone_bytes on evict/promote

Fixes: #7942
Signed-off-by: Samuel Just <sam.just@inktank.com>