overlay2 no longer sets `archive.OverlayWhiteoutFormat` when
running in UserNS, so we can remove the complicated logic in the
archive package.
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
Ubuntu kernel supports overlayfs in user namespaces.
However, Docker had previously crafting overlay opaques directly
using mknod(2) and setxattr(2), which are not supported in userns.
Tested with LXD, Ubuntu 18.04, kernel 4.15.0-36-generic #39-Ubuntu.
Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
