[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>