ReplicatedPG::do_op: use get_object_context for list-snaps

find_object_context provides some niceties which we don't need since we know
the oid of the clones.  Problematically, it also return ENOENT if the snap
requested happens to have been removed.  Even in such a case, the clone may
well still exist for other snaps.  Rather than modify find_object_context to
avoid this situation for this caller, we'll simply do it inline in do_op.

Fixes: #7858
Signed-off-by: Samuel Just <sam.just@inktank.com>