selftests: openat2: fix build error on newer glibc

It appears that newer glibcs check that openat(O_CREAT) was provided a
fourth argument (rather than passing garbage), resulting in the
following build error:

> In file included from /usr/include/fcntl.h:301,
>                  from helpers.c:9:
> In function 'openat',
>     inlined from 'touchat' at helpers.c:49:11:
> /usr/include/x86_64-linux-gnu/bits/fcntl2.h:126:4: error: call to
> '__openat_missing_mode' declared with attribute error: openat with O_CREAT
> or O_TMPFILE in third argument needs 4 arguments
>   126 |    __openat_missing_mode ();
>       |    ^~~~~~~~~~~~~~~~~~~~~~~~

Reported-by: Shuah Khan <shuah@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Tested-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>