Defrag user check in ip_expire was not updated after adding support for
"conntrack zones".

This bug manifests as a RFC violation, since the router will send
the icmp time exceeeded message when using conntrack zones.
Signed-off-by: Vasily Averin <vvs@openvz.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>