dst: Clone child entry in skb_dst_pop

We clone the child entry in skb_dst_pop before we call
skb_dst_drop(). Otherwise we might kill the child right
before we return it to the caller.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>