NetworkPkg/Mtftp6Dxe: Support windowsize in read request operation.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=886

This patch is to support the TFTP windowsize option described in RFC 7440.
The feature allows the client and server to negotiate a window size of
consecutive blocks to send as an alternative for replacing the single-block
lockstep schema.

Currently, the windowsize for write request operation is not supported since
there is no real use cases.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Shao Ming <ming.shao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>

NetworkPkg/Mtftp6Dxe/Mtftp6Wrq.c

@@ -318,7 +318,7 @@ Mtftp6WrqHandleOack (
   }
   ASSERT (Options != NULL);
 
-  Status = Mtftp6ParseExtensionOption (Options, Count, FALSE, &ExtInfo);
+  Status = Mtftp6ParseExtensionOption (Options, Count, FALSE, Instance->Operation, &ExtInfo);
 
   if (EFI_ERROR(Status) || !Mtftp6WrqOackValid (&ExtInfo, &Instance->ExtInfo)) {
     //