netfilter: ipset: Correct rcu_dereference() call in ip_set_put_comment()

The function is called when rcu_read_lock() is held and not
when rcu_read_lock_bh() is held.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>