mm/secretmem: remove reduntant return value

The return value @ret is always 0, so remove it and return 0 directly.

Link: https://lkml.kernel.org/r/20220920012205.246217-1-xiujianfeng@huawei.com
Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>