fib: reduce save_rewrite_length to u8

This is a preparation step for migrating NAT to use SVR (shallow virtual
reassembly) to conserve space in vnet_buffer. Since max rewrite length
is currently pre-data size (128), u8 is sufficient to hold that value.

Type: refactor

Change-Id: I5374bb396e178245b870cb0bbf1370d2a54230bc
Signed-off-by: Klement Sekera <ksekera@cisco.com>
diff --git a/src/vnet/vxlan/vxlan.h b/src/vnet/vxlan/vxlan.h
index 7ade7c0..e8fc15b 100644
--- a/src/vnet/vxlan/vxlan.h
+++ b/src/vnet/vxlan/vxlan.h
@@ -129,7 +129,7 @@
   u32 dev_instance;		/* Real device instance in tunnel vector */
   u32 user_instance;		/* Instance name being shown to user */
 
-    vnet_declare_rewrite (VLIB_BUFFER_PRE_DATA_SIZE);
+    VNET_DECLARE_REWRITE;
 } vxlan_tunnel_t;
 
 #define foreach_vxlan_input_next        \