netfilter: snmp: avoid stack size warning

net/ipv4/netfilter/nf_nat_snmp_basic.c:1158:1: warning: the frame size
of 1160 bytes is larger than 1024 bytes

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