ipv6: stable request

[Denys Zagorui]

12d94a804946af291e24b80fc53ec86264765781 
<https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/commit/?h=v4.14.29&id=12d94a804946af291e24b80fc53ec86264765781> 
ipv6: fix NULL dereference in ip6_route_dev_notify()

This BUG touches 4.1 4.4 4.9.