ksmbd: Use struct_size() helper in ksmbd_negotiate_smb_dialect()

Prefer struct_size() over open-coded versions.

Link: https://github.com/KSPP/linux/issues/160


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Steve French <stfrench@microsoft.com>