netfilter: ipset: move registration message to init from net_init

Commit 1785e8f473 ("netfiler: ipset: Add net namespace for ipset") moved
the initialization print into net_init, which can get called a lot due
to namespaces. Move it back into init, reduce to pr_info.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>