netfilter: connlimit: move lock array out of struct connlimit_data

Eric points out that the locks can be global.
Moreover, both Jesper and Eric note that using only 32 locks increases
false sharing as only two cache lines are used.

This increases locks to 256 (16 cache lines assuming 64byte cacheline and
4 bytes per spinlock).

Suggested-by: Jesper Dangaard Brouer <brouer@redhat.com>
Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>