Use memdup_user_nul() helper instead of open-coding to simplify the code.

Link: https://lkml.kernel.org/r/20220826073337.2085798-1-yangyingliang@huawei.comSigned-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>