dio-writeback-race: fix missing mode in O_CREAT

dio-writeback-race: fix missing mode in O_CREAT

Fix this build error:

In file included from /usr/include/fcntl.h:314,
                 from dio-writeback-race.c:40:
In function 'open',
    inlined from 'main' at dio-writeback-race.c:110:7:
/usr/include/x86_64-linux-gnu/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 ();
      |           ^~~~~~~~~~~~~~~~~~~~~~

Cc: <fstests@vger.kernel.org> # v2025.02.16
Fixes: 17fb49493426ad ("fstests: add a generic test to verify direct IO writes with buffer contents change")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>