overlay: copy up and remove from another namespace

overlay: copy up and remove from another namespace

Via unshare, copy up dir from lower dir then remove
them in another unshared namespace.

This would fail before v4.7 kernel with EPERM.

Kernel commit 3fe6e52f
  ovl: override creds with the ones from the superblock mounter
fixed this issue, with this reproducer provided. Original
reproducer was written by commit author:
Antonio Murdaca <amurdaca@redhat.com>

Signed-off-by: Xiong Zhou <xzhou@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>