Test that openat2() rejects unknown flags in the upper 32 bit range.

Link: https://lore.kernel.org/r/20210528092417.3942079-4-brauner@kernel.org
Cc: Richard Guy Briggs <rgb@redhat.com>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>