xt_qtaguid: check xt_sock socket before releasing

Bug in xt_qtaguid wherein xt_socket_get_*_sk() can steal the sk from
skb without taking a reference, so we may end up lowering the refcnt
of sk too much.

This is hard to trigger, you need to take a special path in tcp_v4, half
closed state + remote has to send you some data packets.

[   74.024530] ------------[ cut here ]------------
[   74.029157] kernel BUG at net/ipv4/inet_timewait_sock.c:101!
[   74.034822] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
[   74.052880] CPU: 0 PID: 3 Comm: ksoftirqd/0 Tainted: P         C O
3.14.13-1.1pre+ #25
[   74.060806] task: cd88f400 ti: cd8a8000 task.ti: cd8a8000
[   74.066219] PC is at __inet_twsk_kill+0x138/0x15c
[   74.070930] LR is at __inet_twsk_kill+0x10c/0x15c
[   74.075639] pc : [<c041efa0>]    lr : [<c041ef74>]    psr: 600f0013
[   74.075639] sp : cd8a9cb0  ip : 000008d8  fp : 00000000
[   74.087128] r10: 00000001  r9 : 000043b0  r8 : cda38ba0
[   74.092357] r7 : 00000001  r6 : 00000001  r5 : 00000002  r4 : c7f50bc0
[   74.098891] r3 : 00000002  r2 : 00000100  r1 : 200f0093  r0 : 00000016
[   74.105426] Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM
Segment kernel
[   74.112742] Control: 30c5387d  Table: 0c6c5e40  DAC: fffffffd
[   74.703277] [<c041efa0>] (__inet_twsk_kill) from [<c04376b4>]
(tcp_timewait_state_process+0x30c/0x380)
[   74.712595] [<c04376b4>] (tcp_timewait_state_process) from
[<c0436d54>] (tcp_v4_rcv+0x17c/0x7c0)
[   74.721391] [<c0436d54>] (tcp_v4_rcv) from [<c04148b8>]
(ip_local_deliver_finish+0xd0/0x2b0)
[   74.729840] [<c04148b8>] (ip_local_deliver_finish) from
[<c0414be0>] (ip_rcv_finish+0x148/0x36c)
[   74.738638] [<c0414be0>] (ip_rcv_finish) from [<c03b9b00>]
(__netif_receive_skb_core+0x200/0x75c)
[   74.747522] [<c03b9b00>] (__netif_receive_skb_core) from
[<c03ba104>] (netif_receive_skb_internal+0x30/0xbc)
[   74.757365] [<c03ba104>] (netif_receive_skb_internal) from
[<c04d30d0>] (brcm_tag_rcv+0x13c/0x178)

Bug (and subsequent fix) reported by an external tester.
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>