xtables-compat: nft-arp: fix warning wrt. sprintf-out-of-bounds

nft-arp.c:112:3: note: 'sprintf' output between 2 and 21 bytes into a destination of size 20
sprintf(buf, "/%s", addr_to_dotted(mask));

Signed-off-by: Florian Westphal <fw@strlen.de>