netfilter: nf_conntrack: ensure that CONNTRACK_LOCKS is power of 2

CONNTRACK_LOCKS is divisor when computer array index, if it is power of
2, compiler will optimize modulo operation as bitwise AND, or else
modulo will lower performance.

Suggested-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>