ksmbd: Use struct_size() to improve get_file_alternate_info()

Use struct_size() to calculate the output buffer length.
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/server/smb2pdu.c

@@ -4883,7 +4883,7 @@ static void get_file_alternate_info(struct ksmbd_work *work,
 	spin_unlock(&dentry->d_lock);
 	file_info->FileNameLength = cpu_to_le32(conv_len);
 	rsp->OutputBufferLength =
-		cpu_to_le32(sizeof(struct smb2_file_alt_name_info) + conv_len);
+		cpu_to_le32(struct_size(file_info, FileName, conv_len));
 }
 
 static int get_file_stream_info(struct ksmbd_work *work,