ieee802154: properly unshare skbs in ieee802154 *_rcv functions

ieee802154 sockets do not properly unshare received skbs, which leads to
panics (at least) when they are used in conjunction with 6lowpan, so
run skb_share_check on received skbs.
6lowpan also contains a use-after-free, which is trivially fixed by
replacing the inlined skb_share_check with the explicit call.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Tested-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ieee802154/dgram.c b/net/ieee802154/dgram.c
index 4c47154..6d251a3 100644
--- a/net/ieee802154/dgram.c
+++ b/net/ieee802154/dgram.c
@@ -329,6 +329,10 @@
 
 static int dgram_rcv_skb(struct sock *sk, struct sk_buff *skb)
 {
+	skb = skb_share_check(skb, GFP_ATOMIC);
+	if (!skb)
+		return NET_RX_DROP;
+
 	if (sock_queue_rcv_skb(sk, skb) < 0) {
 		kfree_skb(skb);
 		return NET_RX_DROP;