Add support for an in-memory routing table cache in order to improve performance when the routing table is retained in a file. The cache holds the most recently used routing table entries and so can eliminate some file access.

Squashed commit of the following:

    net/route:  Flush in cache when any entry is deleted from the routing table.  When a router matching an IP address is found, add the routing table entry to the cache.

    net/route:  Add utility functions to manage an in-memory cache to improve performance when use a file-based routing table.

net/route/cacheroute.h

@@ -0,0 +1,143 @@
+/****************************************************************************
+ * net/route/cacheroute.h
+ *
+ *   Copyright (C) 2017 Gregory Nutt. All rights reserved.
+ *   Author: Gregory Nutt <gnutt@nuttx.org>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name NuttX nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+
+#ifndef __NET_ROUTE_CACHEROUTE_H
+#define __NET_ROUTE_CACHEROUTE_H 1
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <nuttx/config.h>
+
+#include "route/route.h"
+
+#if defined(CONFIG_ROUTE_IPv4_CACHEROUTE) || defined(CONFIG_ROUTE_IPv6_CACHEROUTE)
+
+/****************************************************************************
+ * Public Function Prototypes
+ ****************************************************************************/
+
+/****************************************************************************
+ * Name: net_init_cacheroute
+ *
+ * Description:
+ *   Initialize the in-memory, routing table cache
+ *
+ * Parameters:
+ *   None
+ *
+ * Returned Value:
+ *   None
+ *
+ * Assumptions:
+ *   Called early in initialization so that no special protection is needed.
+ *
+ ****************************************************************************/
+
+void net_init_cacheroute(void);
+
+/****************************************************************************
+ * Name: net_addcache_ipv4 and net_addcache_ipv6
+ *
+ * Description:
+ *   Add one route to the routing table cache
+ *
+ * Parameters:
+ *   None
+ *
+ * Returned Value:
+ *   Zero (OK) is returned on success; a negated errno value is returned
+ *   on any failure.
+ *
+ ****************************************************************************/
+
+#ifdef CONFIG_ROUTE_IPv4_CACHEROUTE
+int net_addcache_ipv4(FAR struct net_route_ipv4_s *route);
+#endif
+
+#ifdef CONFIG_ROUTE_IPv6_CACHEROUTE
+int net_addcache_ipv6(FAR struct net_route_ipv6_s *route);
+#endif
+
+/****************************************************************************
+ * Name: net_foreachcache_ipv4/net_foreachcache_ipv6
+ *
+ * Description:
+ *   Traverse the routing table cahce
+ *
+ * Parameters:
+ *   handler - Will be called for each route in the routing table cache.
+ *   arg     - An arbitrary value that will be passed tot he handler.
+ *
+ * Returned Value:
+ *   Zero (OK) returned if the entire table was searched.  A negated errno
+ *   value will be returned in the event of a failure.  Handlers may also
+ *   terminate the search early with any non-zero value.
+ *
+ ****************************************************************************/
+
+#ifdef CONFIG_ROUTE_IPv4_CACHEROUTE
+int net_foreachcache_ipv4(route_handler_ipv4_t handler, FAR void *arg);
+#endif
+
+#ifdef CONFIG_ROUTE_IPv6_CACHEROUTE
+int net_foreachcache_ipv6(route_handler_ipv6_t handler, FAR void *arg);
+#endif
+
+/****************************************************************************
+ * Name: net_flushcache_ipv4 and net_flushcache_ipv6
+ *
+ * Description:
+ *   Flush the content of the routing table cache
+ *
+ * Parameters:
+ *   None
+ *
+ * Returned Value:
+ *   None
+ *
+ ****************************************************************************/
+
+#ifdef CONFIG_ROUTE_IPv4_CACHEROUTE
+void net_flushcache_ipv4(void);
+#endif
+
+#ifdef CONFIG_ROUTE_IPv6_CACHEROUTE
+void net_flushcache_ipv6(void);
+#endif
+
+#endif /* CONFIG_ROUTE_IPv4_CACHEROUTE || CONFIG_ROUTE_IPv6_CACHEROUTE */
+#endif /* __NET_ROUTE_CACHEROUTE_H */