netfilter: conntrack: fix ipv6 exthdr error check

smatch warnings:
net/netfilter/nf_conntrack_proto.c:167 nf_confirm() warn: unsigned 'protoff' is never less than zero.

We need to check if ipv6_skip_exthdr() returned an error, but protoff is
unsigned.  Use a signed integer for this.

Fixes: a70e4834

 ("netfilter: conntrack: merge ipv4+ipv6 confirm functions")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>