6lowpan: always enable link-layer acknowledgments

This feature is especially important when using fragmentation, because
the reassembly mechanism cannot recover from the loss of a fragment.

Note that some hardware ignore this flag and not will not transmit
acknowledgments even if this is set.
Signed-off-by: Tony Cheneau <tony.cheneau@amnesiak.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ieee802154/6lowpan.c

@@ -584,6 +584,10 @@ static int lowpan_header_create(struct sk_buff *skb,
 
 		mac_cb(skb)->flags = IEEE802154_FC_TYPE_DATA;
 
+		/* request acknowledgment when possible */
+		if (!lowpan_is_addr_broadcast(daddr))
+			mac_cb(skb)->flags |= MAC_CB_FLAG_ACKREQ;
+
 		return dev_hard_header(skb, lowpan_dev_info(dev)->real_dev,
 				type, (void *)&da, (void *)&sa, skb->len);
 	}