ipv6: add complete rcu protection around np->opt

[ Upstream commit 45f6fad84cc305103b28d73482b344d7f5b76f39 ]

This patch addresses multiple problems :

UDP/RAW sendmsg() need to get a stable struct ipv6_txoptions
while socket is not locked : Other threads can change np->opt
concurrently. Dmitry posted a syzkaller
(http://github.com/google/syzkaller

) program desmonstrating
use-after-free.

Starting with TCP/DCCP lockless listeners, tcp_v6_syn_recv_sock()
and dccp_v6_request_recv_sock() also need to use RCU protection
to dereference np->opt once (before calling ipv6_dup_options())

This patch adds full RCU protection to np->opt

b/28746669
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Dennis Cagle <dcagle@codeaurora.org>
Git-commit: 45f6fad84cc305103b28d73482b344d7f5b76f39
Git-repo: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git
(cherry picked from commit 46ddb98e2018a5a62cefa75b3c80882850c91e39)
Change-Id: Ie893308ab0950d8d5494c090e04b8971940a2549