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>

NetworkPkg/IpSecDxe/Ikev2/Exchange.c

@@ -705,7 +705,7 @@ ON_REPLY:
   //
   // Generate the reply packet if needed and send it out.
   //
-  if (IkePacket->Header->Flags != IKE_HEADER_FLAGS_RESPOND) {
+  if (!(IkePacket->Header->Flags & IKE_HEADER_FLAGS_RESPOND)) {
     Reply = mIkev2CreateChild.Generator ((UINT8 *) IkeSaSession, &IkePacket->Header->MessageId);
     if (Reply != NULL) {
       Status = Ikev2SendIkePacket (UdpService, (UINT8 *) &(IkeSaSession->SessionCommon), Reply, 0);