netfilter: nf_ct_timestamp: Fix BUG_ON after netns deletion

When having nf_conntrack_timestamp enabled deleting a netns
can lead to the following BUG being triggered:

[63836.660000] Kernel bug detected[#1]:
[63836.660000] CPU: 0 PID: 0 Comm: swapper Not tainted 3.10.18 #14
[63836.660000] task: 802d9420 ti: 802d2000 task.ti: 802d2000
[63836.660000] $ 0   : 00000000 00000000 00000000 00000000
[63836.660000] $ 4   : 00000001 00000004 00000020 00000020
[63836.660000] $ 8   : 00000000 80064910 00000000 00000000
[63836.660000] $12   : 0bff0002 00000001 00000000 0a0a0abe
[63836.660000] $16   : 802e70a0 85f29d80 00000000 00000004
[63836.660000] $20   : 85fb62a0 00000002 802d3bc0 85fb62a0
[63836.660000] $24   : 00000000 87138110
[63836.660000] $28   : 802d2000 802d3b40 00000014 871327cc
[63836.660000] Hi    : 000005ff
[63836.660000] Lo    : f2edd000
[63836.660000] epc   : 87138794 __nf_ct_ext_add_length+0xe8/0x1ec [nf_conntrack]
[63836.660000]     Not tainted
[63836.660000] ra    : 871327cc nf_conntrack_in+0x31c/...