ReplicatedPG::do_op: don't return ENOENT for whiteout on snapdir read

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG::do_osd_ops: consider head whiteout in list-snaps

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1719 from ceph/wip-8168

Wip 8168

Reviewed-by: Sage Weil <sage@inktank.com>