iptables: Fix and refactor iterate_entries()

iptables: Fix and refactor iterate_entries()

Updating the builtin and hook index is more complex then one
would expect. In order to be able to update them correctly
we need also to pass in the underflow table to the iterate
function. To improve the readability the valid_hook magic
has been moved into next_hook_entry_index() which does exactly
as the name says.