netfilter: nf_queue: relax NFQA_CT attribute check

Allow modifying attributes of the conntrack associated with a packet
without first requesting ct data via CFG_F_CONNTRACK or extra
nfnetlink_conntrack socket.

Also remove unneded rcu_read_lock; the entire function is already
protected by rcu.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>