c8d/list: Handle unpacked layers when calculating shared size

After a535a65 the size reported by the
image list was changed to include all platforms of that image.

This made the "shared size" calculation consider all diff ids of all the
platforms available in the image which caused "snapshot not found"
errors when multiple images were sharing the same layer which wasn't
unpacked. This could happen frequently because by default only the host
platform is unpacked for pulled images.

This was could be reproduced with:
```
$ docker pull docker.io/docker/desktop-kubernetes-coredns:v1.11.1
$ docker pull docker.io/docker/desktop-kubernetes-etcd:3.5.10-0
$ docker system df
```

Which is broken before this commit.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

daemon/containerd/image_list.go

@@ -650,6 +650,11 @@ func computeSharedSize(chainIDs []digest.Digest, layers map[digest.Digest]int, s
 		}
 		size, err := sizeFn(chainID)
 		if err != nil {
+			// Several images might share the same layer and neither of them
+			// might be unpacked (for example if it's a non-host platform).
+			if cerrdefs.IsNotFound(err) {
+				continue
+			}
 			return 0, err
 		}
 		sharedSize += size