[IPV4]: OOPS with NETLINK_FIB_LOOKUP netlink socket

[ Regression added by changeset:
	cd40b7d3
	[NET]: make netlink user -> kernel interface synchronious
  -DaveM ]

nl_fib_input re-reuses incoming skb to send the reply. This means that this
packet will be freed twice, namely in:
- netlink_unicast_kernel
- on receive path
Use clone to send as a cure, the caller is responsible for kfree_skb on error.

Thanks to Alexey Dobryan, who originally found the problem.
Signed-off-by: NDenis V. Lunev <den@openvz.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/fib_frontend.c

@@ -804,10 +804,13 @@ static void nl_fib_input(struct sk_buff *skb)
 
 	nlh = nlmsg_hdr(skb);
 	if (skb->len < NLMSG_SPACE(0) || skb->len < nlh->nlmsg_len ||
-	    nlh->nlmsg_len < NLMSG_LENGTH(sizeof(*frn))) {
-		kfree_skb(skb);
+	    nlh->nlmsg_len < NLMSG_LENGTH(sizeof(*frn)))
 		return;
-	}
+
+	skb = skb_clone(skb, GFP_KERNEL);
+	if (skb == NULL)
+		return;
+	nlh = nlmsg_hdr(skb);
 
 	frn = (struct fib_result_nl *) NLMSG_DATA(nlh);
 	tb = fib_get_table(frn->tb_id_in);