netfilter: nf_log: account for size of NLMSG_DONE attribute

We currently neither account for the nlattr size, nor do we consider
the size of the trailing NLMSG_DONE when allocating nlmsg skb.

This can result in nflog to stop working, as __nfulnl_send() re-tries
sending forever if it failed to append NLMSG_DONE (which will never
work if buffer is not large enough).
Reported-by: Houcheng Lin <houcheng@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nfnetlink_log.c

@@ -649,7 +649,8 @@ nfulnl_log_packet(struct net *net,
 		+ nla_total_size(sizeof(u_int32_t))	/* gid */
 		+ nla_total_size(plen)			/* prefix */
 		+ nla_total_size(sizeof(struct nfulnl_msg_packet_hw))
-		+ nla_total_size(sizeof(struct nfulnl_msg_packet_timestamp));
+		+ nla_total_size(sizeof(struct nfulnl_msg_packet_timestamp))
+		+ nla_total_size(sizeof(struct nfgenmsg));	/* NLMSG_DONE */
 
 	if (in && skb_mac_header_was_set(skb)) {
 		size +=   nla_total_size(skb->dev->hard_header_len)
@@ -692,8 +693,7 @@ nfulnl_log_packet(struct net *net,
 		goto unlock_and_release;
 	}
 
-	if (inst->skb &&
-	    size > skb_tailroom(inst->skb) - sizeof(struct nfgenmsg)) {
+	if (inst->skb && size > skb_tailroom(inst->skb)) {
 		/* either the queue len is too high or we don't have
 		 * enough room in the skb left. flush to userspace. */
 		__nfulnl_flush(inst);