netfilter: conntrack: re-fetch conntrack after insertion

In the Linux kernel, the following vulnerability has been resolved: netfilter: conntrack: re-fetch conntrack after insertion In case the conntrack is clashing, insertion can free skb->_nfct and set skb->_nfct to the already-confirmed entry. This wasn't found before because the conntrack entry and the extension space used to free'd after an rcu grace period, plus the race needs events enabled to trigger.