selftests/kcmp: remove unused open mode

Android bionic warns that open modes are ignored if O_CREAT or O_TMPFILE
aren't specified.  The permissions for the file are set above:

	fd1 = open(kpath, O_RDWR | O_CREAT | O_TRUNC, 0644);

Link: https://lkml.kernel.org/r/20240429234610.191144-1-edliaw@google.com
Fixes: d97b46a6

 ("syscalls, x86: add __NR_kcmp syscall")
Signed-off-by: Edward Liaw <edliaw@google.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>