IPv6 TCP sockets store in np->pktoptions skbs, and use skb_set_owner_r()
to charge the skb to socket.

It means that destructor must be called while socket is locked.

Therefore, we cannot use skb_get() or atomic_inc(&skb->users)
to protect ourselves : kfree_skb() might race with other users
manipulating sk->sk_forward_alloc

Fix this race by holding socket lock for the duration of
ip6_datagram_recv_ctl()
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>