ieee820154: 6lowpan: dispatch evaluation rework

This patch complete reworks the evaluation of 6lowpan dispatch value by
introducing a receive handler mechanism for each dispatch value.

A list of changes:

 - Doing uncompression on-the-fly when FRAG1 is received, this require
   some special handling for 802.15.4 lltype in generic 6lowpan branch
   for setting the payload length correct.
 - Fix dispatch mask for fragmentation.
 - Add IPv6 dispatch evaluation for FRAG1.
 - Add skb_unshare for dispatch which might manipulate the skb data
   buffer.

Cc: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/6lowpan/iphc.c

@@ -366,7 +366,18 @@ lowpan_header_decompress(struct sk_buff *skb, struct net_device *dev,
 			return err;
 	}
 
-	hdr.payload_len = htons(skb->len);
+	switch (lowpan_priv(dev)->lltype) {
+	case LOWPAN_LLTYPE_IEEE802154:
+		if (lowpan_802154_cb(skb)->d_size)
+			hdr.payload_len = htons(lowpan_802154_cb(skb)->d_size -
+						sizeof(struct ipv6hdr));
+		else
+			hdr.payload_len = htons(skb->len);
+		break;
+	default:
+		hdr.payload_len = htons(skb->len);
+		break;
+	}
 
 	pr_debug("skb headroom size = %d, data length = %d\n",
 		 skb_headroom(skb), skb->len);

net/6lowpan/nhc_udp.c

@@ -71,7 +71,18 @@ static int udp_uncompress(struct sk_buff *skb, size_t needed)
 	 * here, we obtain the hint from the remaining size of the
 	 * frame
 	 */
-	uh.len = htons(skb->len + sizeof(struct udphdr));
+	switch (lowpan_priv(skb->dev)->lltype) {
+	case LOWPAN_LLTYPE_IEEE802154:
+		if (lowpan_802154_cb(skb)->d_size)
+			uh.len = htons(lowpan_802154_cb(skb)->d_size -
+				       sizeof(struct ipv6hdr));
+		else
+			uh.len = htons(skb->len + sizeof(struct udphdr));
+		break;
+	default:
+		uh.len = htons(skb->len + sizeof(struct udphdr));
+		break;
+	}
 	pr_debug("uncompressed UDP length: src = %d", ntohs(uh.len));
 
 	/* replace the compressed UDP head by the uncompressed UDP