This function reacquires the rtnl_lock() which is already held by
nf_unregister_hook().

This can be triggered via: modprobe nf_conntrack_ipv4 && rmmod nf_conntrack_ipv4

[  720.628746] INFO: task rmmod:3578 blocked for more than 120 seconds.
[  720.628749]       Not tainted 4.2.0-rc2+ #113
[  720.628752] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  720.628754] rmmod           D ffff8800ca46fd58     0  3578   3571 0x00000080
[...]
[  720.628783] Call Trace:
[  720.628790]  [<ffffffff8152ea0b>] schedule+0x6b/0x90
[  720.628795]  [<ffffffff8152ecb3>] schedule_preempt_disabled+0x13/0x20
[  720.628799]  [<ffffffff8152ff55>] mutex_lock_nested+0x1f5/0x380
[  720.628803]  [<ffffffff81462622>] ? rtnl_lock+0x12/0x20
[  720.628807]  [<ffffffff81462622>] ? rtnl_lock+0x12/0x20
[  720.628812]  [<ffffffff81462622>] rtnl_lock+0x12/0x20
[  720.628817]  [<ffffffff8148ab25>] nf_queue_nf_hook_drop+0x15/0x160
[  720.628825]  [<ffffffff81488d48>] nf_unregister_net_hook+0x168/0x190
[  720.628831]  [<ffffffff81488e24>] nf_unregister_hook+0x64/0x80
[  720.628837]  [<ffffffff81488e60>] nf_unregister_hooks+0x20/0x30
[...]

Moreover, nf_unregister_net_hook() should only destroy the queue for this
netns, not for every netns.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Fixes: 085db2c0

 ("netfilter: Per network namespace netfilter hooks.")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>