xfrm6: Fix a offset value for network header in _decode_session6

When a network-layer header has multiple IPv6 extension headers, then offset
for mobility header goes wrong. This regression breaks an xfrm policy lookup
for a particular receive packet. Binding update packets of Mobile IPv6
are all discarded without this fix.

Fixes: de3b7a06 ("xfrm6: Fix transport header offset in _decode_session6.")
Signed-off-by: Hajime Tazaki <tazaki@sfc.wide.ad.jp>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv6/xfrm6_policy.c

@@ -200,6 +200,7 @@ _decode_session6(struct sk_buff *skb, struct flowi *fl, int reverse)
 
 #if IS_ENABLED(CONFIG_IPV6_MIP6)
 		case IPPROTO_MH:
+			offset += ipv6_optlen(exthdr);
 			if (!onlyproto && pskb_may_pull(skb, nh + offset + 3 - skb->data)) {
 				struct ip6_mh *mh;