netfilter: ip_set: rename nfnl_dereference()/nfnl_set()

The next patch will introduce a nfnl_dereference() macro that actually
checks that the appropriate mutex is held and therefore needs a
subsystem argument.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>