MdeModulePkg: VariableSmmRuntimeDxe: Fix Variable Policy Message Length

MdeModulePkg: VariableSmmRuntimeDxe: Fix Variable Policy Message Length

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3709

In EDKII implementation of variable policy, the DXE runtime agent would
communicate to MM to disable, register or query policies. However, these
operations populate the value of MessageLength that includes communicate
header to include MM communicate header, which mismatches with the
description of PI specification.

This fix will correct the MessageLength field calculation to exclude
the size of MM_COMMUNICATE_HEADER.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Michael Kubacki <michael.kubacki@microsoft.com>
Signed-off-by: Kun Qin <kuqin12@gmail.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>