netfilter: conntrack: comparison of unsigned in cthelper confirmation

net/netfilter/nf_conntrack_core.c: In function nf_confirm_cthelper:
net/netfilter/nf_conntrack_core.c:2117:15: warning: comparison of unsigned expression in < 0 is always false [-Wtype-limits]
 2117 |   if (protoff < 0 || (frag_off & htons(~0x7)) != 0)
      |               ^

ipv6_skip_exthdr() returns a signed integer.

Reported-by: Colin Ian King <colin.king@canonical.com>
Fixes: 703acd70 ("netfilter: nfnetlink_cthelper: unbreak userspace helper support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>