When cloning a rte and replacing its rta, remember to free the old one

so that we don't leak memory. Thanks go to the resource tracking system
for quickly discovering the source of leakages.

filter/filter.c

@@ -215,6 +215,7 @@ rta_cow(void)
     memcpy(f_rta_copy, (*f_rte)->attrs, sizeof(rta));
     f_rta_copy->aflags = 0;
     *f_rte = rte_cow(*f_rte);
+    rta_free((*f_rte)->attrs);
     (*f_rte)->attrs = f_rta_copy;
   }
 }