routecache fails when the cache is full

When the cache is full, route cache erase the last entry from it.
The simple way of doing it correctly is with pop_back.

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

bsd/sys/net/routecache.hh

@@ -108,7 +108,7 @@ class silly_rtable {
 public:
     void add(u32 a, u32 n, const nonlockable_rtentry &r) {
         while (entries.size() >= max_entries) {
-            entries.erase(entries.end());
+            entries.pop_back();
         }
         entries.emplace_front(a, n, r);
     }