ksmbd: switch to use kmemdup_nul() helper

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

Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Steve French <stfrench@microsoft.com>