ksmbd: Use struct_size() helper in ksmbd_negotiate_smb_dialect()

Prefer struct_size() over open-coded versions.

Link: KSPP/linux#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>

Author: GustavoARSilva

fs/smb/server/smb_common.c

@@ -266,7 +266,7 @@ static int ksmbd_negotiate_smb_dialect(void *buf)
 		if (smb2_neg_size > smb_buf_length)
 			goto err_out;
 
-		if (smb2_neg_size + le16_to_cpu(req->DialectCount) * sizeof(__le16) >
+		if (struct_size(req, Dialects, le16_to_cpu(req->DialectCount)) >
 		    smb_buf_length)
 			goto err_out;