netfilter: nfnetlink: use list_for_each_entry_safe to delete all objects

cttimeout and acct objects are deleted from the list while traversing
it, so use list_for_each_entry is unsafe here.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>