netfilter: nf_nat: fix out-of-bounds access in address selection

include/linux/jhash.h:138:16: warning: array subscript is above array bounds
[jhash2() expects the number of u32 in the key]

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>