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>

selftests: add openat2(2) selftests

Test all of the various openat2(2) flags. A small stress-test of a
symlink-rename attack is included to show that the protections against
".."-based attacks are sufficient.

The main things these self-tests are enforcing are:

* The struct+usize ABI for openat2(2) and copy_struct_from_user() to
ensure that upgrades will be handled gracefully (in addition,
ensuring that misaligned structures are also handled correctly).

* The -EINVAL checks for openat2(2) are all correctly handled to avoid
userspace passing unknown or conflicting flag sets (most
importantly, ensuring that invalid flag combinations are checked).

* All of the RESOLVE_* semantics (including errno values) are
correctly handled with various combinations of paths and flags.

* RESOLVE_IN_ROOT correctly protects against the symlink rename(2)
attack that has been responsible for several CVEs (and likely will
be responsible for several more).

Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>