[NETFILTER]: conntrack_netlink: Fix locking during conntrack_create

The current codepath allowed for ip_conntrack_lock to be unlock'ed twice.

Signed-off-by: Pablo Neira <pablo@eurodev.net>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>