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 <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>