ksmbd: remove unsafe_memcpy use in session setup

Kees pointed out to just use directly ->Buffer instead of pointing
->Buffer using offset not to use unsafe_memcpy().

Suggested-by: Kees Cook <kees@kernel.org>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>