openvswitch: Fix IPv6 exthdr handling with ct helpers.

Static code analysis reveals the following bug:

net/openvswitch/conntrack.c:281 ovs_ct_helper()
warn: unsigned 'protoff' is never less than zero.

This signedness bug breaks error handling for IPv6 extension headers when
using conntrack helpers. Fix the error by using a local signed variable.

Fixes: cae3a2627520: "openvswitch: Allow attaching helpers to ct
action"
Reported-by: Dan Carpenter <[e-mail]>
Signed-off-by: Joe Stringer <[e-mail]>
Acked-by: Pravin B Shelar <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/openvswitch/conntrack.c

···
 	case NFPROTO_IPV6: {
 		u8 nexthdr = ipv6_hdr(skb)->nexthdr;
 		__be16 frag_off;
+		int ofs;
 
-		protoff = ipv6_skip_exthdr(skb, sizeof(struct ipv6hdr),
-					   &nexthdr, &frag_off);
-		if (protoff < 0 || (frag_off & htons(~0x7)) != 0) {
+		ofs = ipv6_skip_exthdr(skb, sizeof(struct ipv6hdr), &nexthdr,
+				       &frag_off);
+		if (ofs < 0 || (frag_off & htons(~0x7)) != 0) {
 			pr_debug("proto header not found\n");
 			return NF_ACCEPT;
 		}
+		protoff = ofs;
 		break;
 	}
 	default: