image: use generic decompressor for calculating DiffID
image: use generic decompressor for calculating DiffID

Currently, `image.GetDiffID` cannot calculate DiffID of zstd layers because it
directly uses `compress/gzip` decompressor.
This commit fixes this issue by using the generic decompressor.

Signed-off-by: Kohei Tokunaga <72a466a7d91579e23b8af87d6766ee221241ce87@gmail.com>

containerd/containerd,vdemeester/containerd,estesp/containerd,mikebrow/containerd,vdemeester/containerd,dmcgowan/containerd,estesp/containerd,mikebrow/containerd,thaJeztah/containerd,dmcgowan/containerd,containerd/containerd,thaJeztah/containerd