mac80211: check skb_linearize() return value

The A-MSDU TX code (within TXQs) didn't always check the return value
of skb_linearize() properly, resulting in potentially passing a frag-
list SKB down to the driver even when it said it can't handle it. Fix
that.

Fixes: 6e0456b545 ("mac80211: add A-MSDU tx support")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/tx.c

@@ -1515,8 +1515,12 @@ out:
 	spin_unlock_bh(&fq->lock);
 
 	if (skb && skb_has_frag_list(skb) &&
-	    !ieee80211_hw_check(&local->hw, TX_FRAG_LIST))
-		skb_linearize(skb);
+	    !ieee80211_hw_check(&local->hw, TX_FRAG_LIST)) {
+		if (skb_linearize(skb)) {
+			ieee80211_free_txskb(&local->hw, skb);
+			return NULL;
+		}
+	}
 
 	return skb;
 }