netfilter: use in_dev_for_each_ifa_rcu

Netfilter hooks are always running under rcu read lock, use
the new iterator macro so sparse won't complain once we add
proper __rcu annotations.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>