cifs: Use help macro to get the mid header size

It's better to use MID_HEADER_SIZE because the unfolded expression
too long. No actual functional changes, minor readability improvement.

Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>