Add missing flags argument to open(2) call with O_CREAT.

Some tests fail to compile if _FORTIFY_SOURCE is defined (to any valid
value) (together with -O), resulting in similar error messages such as:

  In file included from /usr/include/fcntl.h:342,
                   from gup_test.c:1:
  In function 'open',
      inlined from 'main' at gup_test.c:206:10:
  /usr/include/bits/fcntl2.h:50:11: error: call to '__open_missing_mode' declared with attribute error: open with O_CREAT or O_TMPFILE in second argument needs 3 arguments
     50 |           __open_missing_mode ();
        |           ^~~~~~~~~~~~~~~~~~~~~~

_FORTIFY_SOURCE is enabled by default in some distributions, so the
tests are not built by default and are skipped.

open(2) man-page warns about missing flags argument: "if it is not
supplied, some arbitrary bytes from the stack will be applied as the
file mode."

Link: https://lkml.kernel.org/r/20240318023445.3192922-1-vt@altlinux.org
Fixes: aeb85ed4 ("tools/testing/selftests/vm/gup_benchmark.c: allow user specified file")
Fixes: fbe37501 ("mm: huge_memory: debugfs for file-backed THP split")
Fixes: c942f5bd ("selftests: soft-dirty: add test for mprotect")
Signed-off-by: Vitaly Chikunov <vt@altlinux.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Keith Busch <kbusch@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>