It is illegal to set skb->sk without corresponding destructor.

Its therefore safe for skb_orphan() to not clear skb->sk if
skb->destructor is not set.

Also avoid clearing skb->destructor if already NULL.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>