netfilter: ipset: Introduce RCU locking in bitmap:* types

There's nothing much required because the bitmap types use atomic
bit operations. However the logic of adding elements slightly changed:
first the MAC address updated (which is not atomic), then the element
activated (added). The extensions may call kfree_rcu() therefore we
call rcu_barrier() at module removal.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>