netfilter: xt_recent: add address masking option

The mask option allows you put all address belonging that mask into
the same recent slot. This can be useful in case that recent is used
to detect attacks from the same network segment.

Tested for backward compatibility.

Signed-off-by: Denys Fedoryshchenko <denys@visp.net.lb>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>