netfilter: ctnetlink: fix rcu context imbalance

Introduced by 7ec47496 (netfilter: ctnetlink: cleanup master conntrack assignation):

net/netfilter/nf_conntrack_netlink.c:1275:2: warning: context imbalance in 'ctnetlink_create_conntrack' - different lock contexts for basic block

Signed-off-by: Patrick McHardy <kaber@trash.net>