selftests: clone3: use SKIP instead of XFAIL

XFAIL is gone since commit 9847d24a ("selftests/harness: Refactor XFAIL
into SKIP"), use SKIP instead.

Fixes: 9847d24a

 ("selftests/harness: Refactor XFAIL into SKIP")
Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>