Skip links to .wh..wh.

When using btrfs or device mapper, we skip all the aufs metadata, starting with `.wh..wh*` ourselves; so the `.wh..wh.plnk` directory is skipped.
We should also skip all the links pointing to this directory.

@alexlarsson could you take a look ? It fixes an issue, but I'm not too familiar with the code. I don't know if we should also add some similar stuff in `archive/changes.go`

ping @jpetazzo @crosbymichael @creack


FYI @dhrp @jlhawn this fixes your issue.