fib: constify the adjacency in the rewrite nodes

Type: refactor

Signed-off-by: Neale Ranns <nranns@cisco.com>
Change-Id: I3aad20b35d89fc541fdf185096d71ca12b09a6e2
diff --git a/src/vnet/ip/ip4_forward.c b/src/vnet/ip/ip4_forward.c
index 44a6819..a67b061 100644
--- a/src/vnet/ip/ip4_forward.c
+++ b/src/vnet/ip/ip4_forward.c
@@ -2423,7 +2423,7 @@
   b = bufs;
   while (n_left_from >= 8)
     {
-      ip_adjacency_t *adj0, *adj1;
+      const ip_adjacency_t *adj0, *adj1;
       ip4_header_t *ip0, *ip1;
       u32 rw_len0, error0, adj_index0;
       u32 rw_len1, error1, adj_index1;