netfilter: pass 'nf_hook_ops' instead of 'list_head' to nf_iterate()

Since 'list_for_each_continue_rcu' has already been replaced by
'list_for_each_entry_continue_rcu', pass 'list_head' to nf_iterate() as a
parameter can not benefit us any more.

This patch will replace 'list_head' with 'nf_hook_ops' as the parameter of
nf_iterate() to save code.

Signed-off-by: Michael Wang <wangyun@linux.vnet.ibm.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>