NetworkPkg/SnpDxe: Register SnpNotifyExitBootServices at TPL_CALLBACK

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1562

The current SnpDxe implementation registers its ExitBootServices event
notification function (SnpNotifyExitBootServices ()) at TPL_NOTIFY. This
function calls PxeShutdown() which issues an UNDI  shutdown operation.
Ultimately, this may invoke Shutdown() in EFI_SIMPLE_NETWORK_PROTOCOL.

The UEFI specification 2.8A Table 27 "TPL Restrictions" restricts the
TPL
for Simple Network Protocol to <= TPL_CALLBACK. In addition, it has been
observed in some 3rd party UNDI drivers to cause an issue further down
the call stack if the TPL is higher than TPL_CALLBACK on invocation.

Therefore, this commit changes the TPL of SnpNotifyExitBootServices() to
TPL_CALLBACK.

Cc: Siyuan Fu <siyuan.fu@intel.com>
Cc: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Maciej Rabeda <maciej.rabeda@linux.intel.com>

NetworkPkg/SnpDxe/Snp.c

@@ -2,6 +2,7 @@
   Implementation of driver entry point and driver binding protocol.
 
 Copyright (c) 2004 - 2019, Intel Corporation. All rights reserved.<BR>
+Copyright (c) Microsoft Corporation.<BR>
 SPDX-License-Identifier: BSD-2-Clause-Patent
 
 **/
@@ -653,7 +654,7 @@ SimpleNetworkDriverStart (
     //
     Status = gBS->CreateEventEx (
                     EVT_NOTIFY_SIGNAL,
-                    TPL_NOTIFY,
+                    TPL_CALLBACK,
                     SnpNotifyExitBootServices,
                     Snp,
                     &gEfiEventExitBootServicesGuid,