netfilter: nf_conncount: restart search when nodes have been erased

Shawn Bohrer reported a following crash:
 |RIP: 0010:rb_erase+0xae/0x360
 [..]
 Call Trace:
  nf_conncount_destroy+0x59/0xc0 [nf_conncount]
  cleanup_match+0x45/0x70 [ip_tables]
  ...

Shawn tracked this down to bogus 'parent' pointer:
Problem is that when we insert a new node, then there is a chance that
the 'parent' that we found was also passed to tree_nodes_free() (because
that node was empty) for erase+free.

Instead of trying to be clever and detect when this happens, restart
the search if we have evicted one or more nodes.  To prevent frequent
restarts, do not perform gc on the second round.

Also, unconditionally schedule the gc worker.
The condition

  gc_count > ARRAY_SIZE(gc_nodes))

cannot be true unless tree grows very large, as the height of the tree
will be low even with hundreds of nodes present.

Fixes: 5c789e13

 ("netfilter: nf_conncount: Add list lock and gc worker, and RCU for init tree search")
Reported-by: Shawn Bohrer <sbohrer@cloudflare.com>
Reviewed-by: Shawn Bohrer <sbohrer@cloudflare.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>