netfilter: xt_hashlimit: fix proc entry leak in netns destroy path

In (32263dd1 netfilter: xt_hashlimit: fix namespace destroy path)
the hashlimit_net_exit() function is always called right before
hashlimit_mt_destroy() to release netns data. If you use xt_hashlimit
with IPv4 and IPv6 together, this produces the following splat via
netconsole in the netns destroy path:

 Pid: 9499, comm: kworker/u:0 Tainted: G        WC O 3.2.0-5-netctl-amd64-core2
 Call Trace:
  [<ffffffff8104708d>] ? warn_slowpath_common+0x78/0x8c
  [<ffffffff81047139>] ? warn_slowpath_fmt+0x45/0x4a
  [<ffffffff81144a99>] ? remove_proc_entry+0xd8/0x22e
  [<ffffffff810ebbaa>] ? kfree+0x5b/0x6c
  [<ffffffffa043c501>] ? hashlimit_net_exit+0x45/0x8d [xt_hashlimit]
  [<ffffffff8128ab30>] ? ops_exit_list+0x1c/0x44
  [<ffffffff8128b28e>] ? cleanup_net+0xf1/0x180
  [<ffffffff810369fc>] ? should_resched+0x5/0x23
  [<ffffffff8105b8f9>] ? process_one_work+0x161/0x269
  [<ffffffff8105aea5>] ? cwq_activate_delayed_work+0x3c/0x48
  [<ffffffff8105c8c2>] ? worker_thread+0xc2/0x145
  [<ffffffff8105c800>] ? manage_workers.isra.25+0x15b/0x15b
  [<ffffffff8105fa01>] ? kthread+0x76/0x7e
  [<ffffffff813581f4>] ? kernel_thread_helper+0x4/0x10
  [<ffffffff8105f98b>] ? kthread_worker_fn+0x139/0x139
  [<ffffffff813581f0>] ? gs_change+0x13/0x13
 ---[ end trace d8c3cc0ad163ef79 ]---
 ------------[ cut here ]------------
 WARNING: at /usr/src/linux-3.2.52/debian/build/source_netctl/fs/proc/generic.c:849
 remove_proc_entry+0x217/0x22e()
 Hardware name:
 remove_proc_entry: removing non-empty directory 'net/ip6t_hashlimit', leaking at least 'IN-REJECT'

This is due to lack of removal net/ip6t_hashlimit/* entries in
hashlimit_proc_net_exit(), since only IPv4 entries are deleted. Fix
it by always removing the IPv4 and IPv6 entries and their parent
directories in the netns destroy path.

Signed-off-by: Sergey Popovich <popovich_sergei@mail.ru>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>