CIFS: convert to use le32_add_cpu()

Convert cpu_to_le32(le32_to_cpu(E1) + E2) to use le32_add_cpu().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Steve French <smfrench@gmail.com>