netfilter: ipset: bitmap:ip,mac: fix listing with timeout

The type when timeout support was enabled, could not list all elements,
just the first ones which could fit into one netlink message: it just
did not continue listing after the first message.

Reported-by: Yoann JUET <yoann.juet@univ-nantes.fr>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Tested-by: Yoann JUET <yoann.juet@univ-nantes.fr>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>