netlink: don't orphan skb in netlink_trim()

Netlink doesn't account skbs to the sending socket, so the there's no
need to orphan the skb before trimming it.

Removing the skb_orphan() call is required for mmap'ed netlink, which uses
a netlink specific skb destructor that must not be invoked before the
final freeing of the skb.

Signed-off-by: Patrick McHardy <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/netlink/af_netlink.c

···
 {
 	int delta;
 
-	skb_orphan(skb);
+	WARN_ON(skb->sk != NULL);
 
 	delta = skb->end - skb->tail;
 	if (delta * 2 < skb->truesize)