netfilter: xt_connlimit: remove mask argument

Instead of passing mask to all the helpers, just fixup the search key
early.

After rbtree conversion, each rbtree node stores connections of same
'addr & mask', so no need to pass the mask too.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>