netfilter: nat: Do not use ARRAY_SIZE() on spinlocks to fix zero div

If no spinlock debugging options (CONFIG_GENERIC_LOCKBREAK,
CONFIG_DEBUG_SPINLOCK, CONFIG_DEBUG_LOCK_ALLOC) are enabled on a UP
platform (e.g. m68k defconfig), arch_spinlock_t is an empty struct,
hence using ARRAY_SIZE(nf_nat_locks) causes a division by zero:

    net/netfilter/nf_nat_core.c: In function ‘nf_nat_setup_info’:
    net/netfilter/nf_nat_core.c:432: warning: division by zero
    net/netfilter/nf_nat_core.c: In function ‘__nf_nat_cleanup_conntrack’:
    net/netfilter/nf_nat_core.c:535: warning: division by zero
    net/netfilter/nf_nat_core.c:537: warning: division by zero
    net/netfilter/nf_nat_core.c: In function ‘nf_nat_init’:
    net/netfilter/nf_nat_core.c:810: warning: division by zero
    net/netfilter/nf_nat_core.c:811: warning: division by zero
    net/netfilter/nf_nat_core.c:824: warning: division by zero

Fix this by using the CONNTRACK_LOCKS definition instead.

Suggested-by: Florian Westphal <fw@strlen.de>
Fixes: 8073e960 ("netfilter: nat: use keyed locks")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>