MdeModulePkg: Add new macros and refine codes

Add 2 macros inNetLib.h
#define  IP4_MASK_MAX          32
#define  IP6_PREFIX_MAX        128
we will use these two macros to check the max mask/prefix length,
instead of
#define  IP4_MASK_NUM          33
#define  IP6_PREFIX_NUM        129
which means a valid number.
This will make the code readability and maintainability.

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

MdeModulePkg/Universal/Network/Ip4Dxe/Ip4Config2Impl.c

@@ -416,7 +416,7 @@ Ip4Config2BuildDefaultRouteTable (
   //
   Count = 0;
 
-  for (Index = IP4_MASK_NUM - 1; Index >= 0; Index--) {
+  for (Index = IP4_MASK_MAX; Index >= 0; Index--) {
 
     NET_LIST_FOR_EACH (Entry, &(IpSb->DefaultRouteTable->RouteArea[Index])) {
       RtEntry = NET_LIST_USER_STRUCT (Entry, IP4_ROUTE_ENTRY, Link);