[arp] Try to avoid discarding ARP cache entries

Discarding the active ARP cache entry in the middle of a download will
substantially disrupt the TCP stream.  Try to minimise any such
disruption by treating ARP cache entries as expensive, and discarding
them only when nothing else is available to discard.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp.c

@@ -1315,7 +1315,7 @@ static unsigned int tcp_discard ( void ) {
 }
 
 /** TCP cache discarder */
-struct cache_discarder tcp_cache_discarder __cache_discarder = {
+struct cache_discarder tcp_discarder __cache_discarder ( CACHE_NORMAL ) = {
 	.discard = tcp_discard,
 };