flow_dissector: Fix fragment handling for header length computation

It turns out that for IPv4 we were reporting the ip_proto of the fragment,
and for IPv6 we were not.  This patch updates that behavior so that we
always report the IP protocol of the fragment.  In addition it takes the
steps of updating the payload offset code so that we will determine the
start of the payload not including the L4 header for any fragment after the
first.
Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Acked-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/flow_dissector.c

@@ -448,14 +448,13 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
 		key_control->flags |= FLOW_DIS_IS_FRAGMENT;
 
 		nhoff += sizeof(_fh);
+		ip_proto = fh->nexthdr;
 
 		if (!(fh->frag_off & htons(IP6_OFFSET))) {
 			key_control->flags |= FLOW_DIS_FIRST_FRAG;
-			if (flags & FLOW_DISSECTOR_F_PARSE_1ST_FRAG) {
-				ip_proto = fh->nexthdr;
+			if (flags & FLOW_DISSECTOR_F_PARSE_1ST_FRAG)
 				goto ip_proto_again;
 		}
-		}
 		goto out_good;
 	}
 	case IPPROTO_IPIP:
@@ -741,6 +740,11 @@ u32 __skb_get_poff(const struct sk_buff *skb, void *data,
 {
 	u32 poff = keys->control.thoff;
 
+	/* skip L4 headers for fragments after the first */
+	if ((keys->control.flags & FLOW_DIS_IS_FRAGMENT) &&
+	    !(keys->control.flags & FLOW_DIS_FIRST_FRAG))
+		return poff;
+
 	switch (keys->basic.ip_proto) {
 	case IPPROTO_TCP: {
 		/* access doff as u8 to avoid unaligned access */