overlay: add tests for data-only redirect with userxattr

overlay: add tests for data-only redirect with userxattr

New kernel feature (target release is v6.16) allows data-only redirect to
be enabled without metacopy and redirect_dir turned on.  This works with or
without verity enabled.

Tests are done with the userxattr option, to verify that it will work in a
user namespace.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>