ipv6: do not clear pinet6 field

We have seen multiple NULL dereferences in __inet6_lookup_established()

After analysis, I found that inet6_sk() could be NULL while the
check for sk_family == AF_INET6 was true.

Bug was added in linux-2.6.29 when RCU lookups were introduced in UDP
and TCP stacks.

Once an IPv6 socket, using SLAB_DESTROY_BY_RCU is inserted in a hash
table, we no longer can clear pinet6 field.

This patch extends logic used in commit fcbdf09d96
("net: fix nulls list corruptions in sk_prot_alloc")

TCP/UDP/UDPLite IPv6 protocols provide their own .clear_sk() method
to make sure we do not clear pinet6 field.

At socket clone phase, we do not really care, as cloning the parent (non
NULL) pinet6 is not adding a fatal race.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/udp_impl.h

@@ -31,6 +31,8 @@ extern int	udpv6_recvmsg(struct kiocb *iocb, struct sock *sk,
 extern int	udpv6_queue_rcv_skb(struct sock * sk, struct sk_buff *skb);
 extern void	udpv6_destroy_sock(struct sock *sk);
 
+extern void udp_v6_clear_sk(struct sock *sk, int size);
+
 #ifdef CONFIG_PROC_FS
 extern int	udp6_seq_show(struct seq_file *seq, void *v);
 #endif