netfilter: ipset: do not call ipset_nest_end after nla_nest_cancel

netfilter: ipset: do not call ipset_nest_end after nla_nest_cancel

In the error handling block, nla_nest_cancel(skb, atd) is called to
cancel the nest operation. But then, ipset_nest_end(skb, atd) is
unexpected called to end the nest operation. This patch calls the
ipset_nest_end only on the branch that nla_nest_cancel is
not called.

Fixes: 45040978c89("netfilter: ipset: Fix set:list type crash when
flush/dump set in parallel")

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>