netfilter: conntrack: Fix UP builds

ARRAY_SIZE(nf_conntrack_locks) is undefined if spinlock_t is an
empty structure. Replace it by CONNTRACK_LOCKS

Fixes: 93bb0ceb

 ("netfilter: conntrack: remove central spinlock nf_conntrack_lock")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>