esp6: support ipv6 nexthdrs process for beet gso segment

For beet mode, when it's ipv6 inner address with nexthdrs set,
the packet format might be:

    ----------------------------------------------------
    | outer  |     | dest |     |      |  ESP    | ESP |
    | IP6 hdr| ESP | opts.| TCP | Data | Trailer | ICV |
    ----------------------------------------------------

Before doing gso segment in xfrm6_beet_gso_segment(), it should
skip all nexthdrs and get the real transport proto, and set
transport_header properly.

This patch is to fix it by simply calling ipv6_skip_exthdr()
in xfrm6_beet_gso_segment().

v1->v2:
  - remove skb_transport_offset(), as it will always return 0
    in xfrm6_beet_gso_segment(), thank Sabrina's check.

Fixes: 7f9e40eb ("esp6: add gso_segment for esp6 beet mode")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv6/esp6_offload.c

@@ -171,7 +171,7 @@ static struct sk_buff *xfrm6_beet_gso_segment(struct xfrm_state *x,
 	struct xfrm_offload *xo = xfrm_offload(skb);
 	struct sk_buff *segs = ERR_PTR(-EINVAL);
 	const struct net_offload *ops;
-	int proto = xo->proto;
+	u8 proto = xo->proto;
 
 	skb->transport_header += x->props.header_len;
 
@@ -182,7 +182,12 @@ static struct sk_buff *xfrm6_beet_gso_segment(struct xfrm_state *x,
 		proto = ph->nexthdr;
 	}
 
-	if (x->sel.family != AF_INET6) {
+	if (x->sel.family == AF_INET6) {
+		__be16 frag;
+
+		skb->transport_header +=
+			ipv6_skip_exthdr(skb, 0, &proto, &frag);
+	} else {
 		skb->transport_header -=
 			(sizeof(struct ipv6hdr) - sizeof(struct iphdr));