mac802154: fix mac header length check

I got report about that sometimes the WARN_ON occurs there which should
never happen. I came to the conclusion that the mac header is there but
inside the headroom of skb. The skb->len information doesn't contain the
information about the headroom length and skb->len is lesser than two.

We check now if the skb_mac_header pointer is set and the room between
mac header pointer and tail pointer.

Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

include/net/mac802154.h

@@ -247,8 +247,9 @@ struct ieee802154_ops {
  */
 static inline __le16 ieee802154_get_fc_from_skb(const struct sk_buff *skb)
 {
-	/* return some invalid fc on failure */
-	if (unlikely(skb->len < 2)) {
+	/* check if we can fc at skb_mac_header of sk buffer */
+	if (unlikely(!skb_mac_header_was_set(skb) ||
+		     (skb_tail_pointer(skb) - skb_mac_header(skb)) < 2)) {
 		WARN_ON(1);
 		return cpu_to_le16(0);
 	}