xtables-compat: fix snprintf truncation warnings

proto is u16 in the data structure, so this gave:
nft-ipv6.c:422:44: warning: '__builtin___snprintf_chk' output may be truncated before the last format character [-Wformat-truncation=]

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