When copying up files and reading directory contents ensure the mounter has
permissions for the operation over the consituent parts (lower and upper).
Where we are in a namespace this ensures that the mounter (root in that
namespace) has permissions over the files and directories, preventing
exposure of protected files and directory contents.

CVE-2015-1328
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>