net: inetdevice: provide replacement iterators for in_ifaddr walk

The ifa_list is protected either by rcu or rtnl lock, but the
current iterators do not account for this.

This adds two iterators as replacement, a later patch in
the series will update them with the needed rcu/rtnl_dereference calls.

Its not done in this patch yet to avoid sparse warnings -- the fields
lack the proper __rcu annotation.

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