mt76: dma: do not report truncated frames to mac80211

Commit b102f0c5 ("mt76: fix array overflow on receiving too many
fragments for a packet") fixes a possible OOB access but it introduces a
memory leak since the pending frame is not released to page_frag_cache
if the frag array of skb_shared_info is full. Commit 93a1d479
("mt76: dma: fix a possible memory leak in mt76_add_fragment()") fixes
the issue but does not free the truncated skb that is forwarded to
mac80211 layer. Fix the leftover issue discarding even truncated skbs.

Fixes: 93a1d479 ("mt76: dma: fix a possible memory leak in mt76_add_fragment()")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/a03166fcc8214644333c68674a781836e0f57576.1612697217.git.lorenzo@kernel.org

drivers/net/wireless/mediatek/mt76/dma.c

@@ -512,13 +512,13 @@ mt76_add_fragment(struct mt76_dev *dev, struct mt76_queue *q, void *data,
 {
 	struct sk_buff *skb = q->rx_head;
 	struct skb_shared_info *shinfo = skb_shinfo(skb);
+	int nr_frags = shinfo->nr_frags;
 
-	if (shinfo->nr_frags < ARRAY_SIZE(shinfo->frags)) {
+	if (nr_frags < ARRAY_SIZE(shinfo->frags)) {
 		struct page *page = virt_to_head_page(data);
 		int offset = data - page_address(page) + q->buf_offset;
 
-		skb_add_rx_frag(skb, shinfo->nr_frags, page, offset, len,
-				q->buf_size);
+		skb_add_rx_frag(skb, nr_frags, page, offset, len, q->buf_size);
 	} else {
 		skb_free_frag(data);
 	}
@@ -527,7 +527,10 @@ mt76_add_fragment(struct mt76_dev *dev, struct mt76_queue *q, void *data,
 		return;
 
 	q->rx_head = NULL;
+	if (nr_frags < ARRAY_SIZE(shinfo->frags))
 		dev->drv->rx_skb(dev, q - dev->q_rx, skb);
+	else
+		dev_kfree_skb(skb);
 }
 
 static int