ReplicatedPG: ensure clones are readable after find_object_context

We only get EAGAIN if the object is missing.  We also need the
clone to be readable if we are reading it.

The other find_object_context callers already require !degraded.

Fixes: #7624
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1386 from ceph/wip-7624

ReplicatedPG: ensure clones are readable after find_object_context

Reviewed-by: Sage Weil <sage@inktank.com>