[NETFILTER]: ULOG/nfnetlink_log: Use better default value for 'nlbufsiz'

Performance tests showed that ULOG may fail on heavy loaded systems
because of failed order-N allocations (N >= 1).

The default value of 4096 is not optimal in the sense that it actually
allocates _two_ contigous physical pages.  Reasoning: ULOG uses
alloc_skb(), which adds another ~300 bytes for skb_shared_info.

This patch sets the default value to NLMSG_GOODSIZE and adds some
documentation at the top.

Signed-off-by: Holger Eitzenberger <heitzenberger@astaro.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/netfilter/nfnetlink_log.c b/net/netfilter/nfnetlink_log.c
index e10512e..50787af 100644
--- a/net/netfilter/nfnetlink_log.c
+++ b/net/netfilter/nfnetlink_log.c
@@ -37,7 +37,7 @@
 #include "../bridge/br_private.h"
 #endif
 
-#define NFULNL_NLBUFSIZ_DEFAULT	4096
+#define NFULNL_NLBUFSIZ_DEFAULT	NLMSG_GOODSIZE
 #define NFULNL_TIMEOUT_DEFAULT 	100	/* every second */
 #define NFULNL_QTHRESH_DEFAULT 	100	/* 100 packets */