netfilter: ctnetlink: fix missing locking while changing conntrack from nfqueue

Since 9cb017665 netfilter: add glue code to integrate nfnetlink_queue and
ctnetlink, we can modify the conntrack entry via nfnl_queue. However, the
change of the conntrack entry via nfnetlink_queue requires appropriate
locking to avoid concurrent updates.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>