6lowpan_rtnl: fix fragmentation with two fragments

This patch fix the 6LoWPAN fragmentation for the case if we have exactly
two fragments. The problem is that the (skb_unprocessed >= frag_cap)
condition is always false on the second fragment after sending the first
fragment. A fragmentation with only one fragment doesn't make any sense.
The solution is that we use a do while loop here, that ensures we sending
always a minimum of two fragments if we need a fragmentation.

This issue was introduced by commit d4b2816d
("6lowpan: fix fragmentation").
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ieee802154/6lowpan_rtnl.c

@@ -312,7 +312,7 @@ lowpan_xmit_fragmented(struct sk_buff *skb, struct net_device *dev,
 	frag_hdr[0] |= LOWPAN_DISPATCH_FRAGN;
 	frag_cap = round_down(payload_cap - LOWPAN_FRAGN_HEAD_SIZE, 8);
 
-	while (skb_unprocessed >= frag_cap) {
+	do {
 		dgram_offset += frag_len;
 		skb_offset += frag_len;
 		skb_unprocessed -= frag_len;
@@ -328,7 +328,7 @@ lowpan_xmit_fragmented(struct sk_buff *skb, struct net_device *dev,
 				 __func__, frag_tag, skb_offset);
 			goto err;
 		}
-	}
+	} while (skb_unprocessed >= frag_cap);
 
 	consume_skb(skb);
 	return NET_XMIT_SUCCESS;