netfilter: make it safer during the inet6_dev->addr_list traversal

inet6_dev->addr_list is protected by inet6_dev->lock, so only using
rcu_read_lock is not enough, we should acquire read_lock_bh(&idev->lock)
before the inet6_dev->addr_list traversal.

Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>