overlay: Add tests for nesting private xattrs

overlay: Add tests for nesting private xattrs

If overlayfs xattr escaping is supported, ensure:
 * We can create "overlay.*" xattrs on a file in the overlayfs
 * We can create an xwhiteout file in the overlayfs

We check for nesting support by trying to getattr an "overlay.*" xattr
in an overlayfs mount, which will return ENOTSUPP in older kernels.

Signed-off-by: Alexander Larsson <alexl@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>