netfilter: conntrack: Remove unused net in nf_conntrack_double_lock()

Since commit a3efd812 ("netfilter: conntrack: move generation
seqcnt out of netns_ct") this param is unused.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>