NetworkPkg/IpSecDxe: Fix wrong IKE header "FLAG" update

NetworkPkg/IpSecDxe: Fix wrong IKE header "FLAG" update

*v2: update the commit log and refine the code comments.

There are three kinds of IKE Exchange process:
#1. Initial Exchange
#2. CREATE_CHILD_SA_Exchange
#3. Information Exchange

The IKE header "FLAG" update is incorrect in #2 and #3 exchange,
which may cause the continue session failure. This patch is used
to correct the updates of IKE header "FLAG" according the RFC4306
section 3.1.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Zhang Lubo <lubo.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiaxin Wu <jiaxin.wu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>