netfilter: nat: remove unnecessary rcu_read_lock in nf_nat_redirect_ipv{4/6}

nf_nat_redirect_ipv4() and nf_nat_redirect_ipv6() are only called by
netfilter hook point. so that rcu_read_lock and rcu_read_unlock() are
unnecessary.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>