netfilter: connlimit: make same_source_net signed

currently returns 1 if they're the same.  Make it work like mem/strcmp
so it can be used as rbtree search function.

Reviewed-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>