netfilter: conntrack: fix infinite loop on rmmod

'rmmod nf_conntrack' can hang forever, because the netns exit
gets stuck in nf_conntrack_cleanup_net_list():

i_see_dead_people:
 busy = 0;
 list_for_each_entry(net, net_exit_list, exit_list) {
  nf_ct_iterate_cleanup(kill_all, net, 0, 0);
  if (atomic_read(&net->ct.count) != 0)
   busy = 1;
 }
 if (busy) {
  schedule();
  goto i_see_dead_people;
 }

When nf_ct_iterate_cleanup iterates the conntrack table, all nf_conn
structures can be found twice:
once for the original tuple and once for the conntracks reply tuple.

get_next_corpse() only calls the iterator when the entry is
in original direction -- the idea was to avoid unneeded invocations
of the iterator callback.

When support for clashing entries was added, the assumption that
all nf_conn objects are added twice, once in original, once for reply
tuple no longer holds -- NF_CLASH_BIT entries are only added in
the non-clashing reply direction.

Thus, if at least one NF_CLASH entry is in the list then
nf_conntrack_cleanup_net_list() always skips it completely.

During normal netns destruction, this causes a hang of several
seconds, until the gc worker removes the entry (NF_CLASH entries
always have a 1 second timeout).

But in the rmmod case, the gc worker has already been stopped, so
ct.count never becomes 0.

We can fix this in two ways:

1. Add a second test for CLASH_BIT and call iterator for those
   entries as well, or:
2. Skip the original tuple direction and use the reply tuple.

2) is simpler, so do that.

Fixes: 6a757c07

 ("netfilter: conntrack: allow insertion of clashing entries")
Reported-by: Chen Yi <yiche@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>