netfilter: nf_qeueue: Drop queue entries on nf_unregister_hook

netfilter: nf_qeueue: Drop queue entries on nf_unregister_hook

Add code to nf_unregister_hook to flush the nf_queue when a hook is
unregistered.  This guarantees that the pointer that the nf_queue code
retains into the nf_hook list will remain valid while a packet is
queued.

I tested what would happen if we do not flush queued packets and was
trivially able to obtain the oops below.  All that was required was
to stop the nf_queue listening process, to delete all of the nf_tables,
and to awaken the nf_queue listening process.

> BUG: unable to handle kernel paging request at 0000000100000001
> IP: [<0000000100000001>] 0x100000001
> PGD b9c35067 PUD 0
> Oops: 0010 [#1] SMP
> Modules linked in:
> CPU: 0 PID: 519 Comm: lt-nfqnl_test Not tainted
> task: ffff8800b9c8c050 ti: ffff8800ba9d8000 task.ti: ffff8800ba9d8000
> RIP: 0010:[<0000000100000001>]  [<0000000100000001>] 0x100000001
> RSP: 0018:ffff8800ba9dba40  EFLAGS: 00010a16
> RAX: ffff8800bab48a00 RBX: ffff8800ba9dba90 RCX: ffff8800ba9dba90
> RDX: ffff8800b9c10128 RSI: ffff8800ba940900 RDI: ffff8800bab48a00
> RBP: ffff8800b9c10128 R08: ffffffff82976660 R09: ffff8800ba9dbb28
> R10: dead000000100100 R11: dead000000200200 R12: ffff8800ba940900
> R13: ffffffff8313fd50 R14: ffff8800b9c95200 R15: 0000000000000000
> FS:  00007fb91fc34700(0000) GS:ffff8800bfa00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000100000001 CR3: 00000000babfb000 CR4: 00000000000007f0
> Stack:
>  ffffffff8206ab0f ffffffff82982240 ffff8800bab48a00 ffff8800b9c100a8
>  ffff8800b9c10100 0000000000000001 ffff8800ba940900 ffff8800b9c10128
>  ffffffff8206bd65 ffff8800bfb0d5e0 ffff8800bab48a00 0000000000014dc0
> Call Trace:
>  [<ffffffff8206ab0f>] ? nf_iterate+0x4f/0xa0
>  [<ffffffff8206bd65>] ? nf_reinject+0x125/0x190
>  [<ffffffff8206dee5>] ? nfqnl_recv_verdict+0x255/0x360
>  [<ffffffff81386290>] ? nla_parse+0x80/0xf0
>  [<ffffffff8206c42c>] ? nfnetlink_rcv_msg+0x13c/0x240
>  [<ffffffff811b2fec>] ? __memcg_kmem_get_cache+0x4c/0x150
>  [<ffffffff8206c2f0>] ? nfnl_lock+0x20/0x20
>  [<ffffffff82068159>] ? netlink_rcv_skb+0xa9/0xc0
>  [<ffffffff820677bf>] ? netlink_unicast+0x12f/0x1c0
>  [<ffffffff82067ade>] ? netlink_sendmsg+0x28e/0x650
>  [<ffffffff81fdd814>] ? sock_sendmsg+0x44/0x50
>  [<ffffffff81fde07b>] ? ___sys_sendmsg+0x2ab/0x2c0
>  [<ffffffff810e8f73>] ? __wake_up+0x43/0x70
>  [<ffffffff8141a134>] ? tty_write+0x1c4/0x2a0
>  [<ffffffff81fde9f4>] ? __sys_sendmsg+0x44/0x80
>  [<ffffffff823ff8d7>] ? system_call_fastpath+0x12/0x6a
> Code:  Bad RIP value.
> RIP  [<0000000100000001>] 0x100000001
>  RSP <ffff8800ba9dba40>
> CR2: 0000000100000001
> ---[ end trace 08eb65d42362793f ]---

Cc: stable@vger.kernel.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>