netfilter: connlimit: use keyed locks

connlimit currently suffers from spinlock contention, example for
4-core system with rps enabled:

+  20.84%   ksoftirqd/2  [kernel.kallsyms] [k] _raw_spin_lock_bh
+  20.76%   ksoftirqd/1  [kernel.kallsyms] [k] _raw_spin_lock_bh
+  20.42%   ksoftirqd/0  [kernel.kallsyms] [k] _raw_spin_lock_bh
+   6.07%   ksoftirqd/2  [nf_conntrack]    [k] ____nf_conntrack_find
+   6.07%   ksoftirqd/1  [nf_conntrack]    [k] ____nf_conntrack_find
+   5.97%   ksoftirqd/0  [nf_conntrack]    [k] ____nf_conntrack_find
+   2.47%   ksoftirqd/2  [nf_conntrack]    [k] hash_conntrack_raw
+   2.45%   ksoftirqd/0  [nf_conntrack]    [k] hash_conntrack_raw
+   2.44%   ksoftirqd/1  [nf_conntrack]    [k] hash_conntrack_raw

May allow parallel lookup/insert/delete if the entry is hashed to
another slot.  With patch:

+  20.95%  ksoftirqd/0  [nf_conntrack] [k] ____nf_conntrack_find
+  20.50%  ksoftirqd/1  [nf_conntrack] [k] ____nf_conntrack_find
+  20.27%  ksoftirqd/2  [nf_conntrack] [k] ____nf_conntrack_find
+   5.76%  ksoftirqd/1  [nf_conntrack] [k] hash_conntrack_raw
+   5.39%  ksoftirqd/2  [nf_conntrack] [k] hash_conntrack_raw
+   5.35%  ksoftirqd/0  [nf_conntrack] [k] hash_conntrack_raw
+   2.00%  ksoftirqd/1  [kernel.kallsyms] [k] __rcu_read_unlock

Improved rx processing rate from ~35kpps to ~50 kpps.

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>